diff --git a/adler32.c b/adler32.c
index a061ee6..4441cfc 100644
--- a/adler32.c
+++ b/adler32.c
@@ -1,21 +1,29 @@
 /*
  * This file is part of the Aaru Data Preservation Suite.
  * Copyright (c) 2019-2021 Natalia Portillo.
- *
- * This library is free software; you can redistribute it and/or modify
- * it under the terms of the GNU Lesser General Public License as
- * published by the Free Software Foundation; either version 2.1 of the
- * License, or (at your option) any later version.
- *
- * This library is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- * Lesser General Public License for more details.
- *
- * You should have received a copy of the GNU Lesser General Public
- * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
 
+/* adler32.c -- compute the Adler-32 checksum of a data stream
+   Copyright (C) 1995-2011 Mark Adler
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+
+  Jean-loup Gailly        Mark Adler
+  jloup@gzip.org          madler@alumni.caltech.edu
+*/
+
 #include <stdint.h>
 #include <stdlib.h>
 
@@ -38,15 +46,138 @@ AARU_EXPORT adler32_ctx* AARU_CALL adler32_init()
 
 AARU_EXPORT int AARU_CALL adler32_update(adler32_ctx* ctx, const uint8_t* data, uint32_t len)
 {
-    uint32_t i;
     if(!ctx || !data) return -1;
 
-    for(i = 0; i < len; i++)
+    uint32_t sum1 = ctx->sum1;
+    uint32_t sum2 = ctx->sum2;
+    unsigned n;
+
+    /* in case user likes doing a byte at a time, keep it fast */
+    if(len == 1)
     {
-        ctx->sum1 = (ctx->sum1 + data[i]) % ADLER_MODULE;
-        ctx->sum2 = (ctx->sum2 + ctx->sum1) % ADLER_MODULE;
+        sum1 += data[0];
+        if(sum1 >= ADLER_MODULE) sum1 -= ADLER_MODULE;
+        sum2 += sum1;
+        if(sum2 >= ADLER_MODULE) sum2 -= ADLER_MODULE;
+
+        ctx->sum1 = sum1 & 0xFFFF;
+        ctx->sum2 = sum2 & 0xFFFF;
+        return 0;
     }
 
+    /* in case short lengths are provided, keep it somewhat fast */
+    if(len < 16)
+    {
+        while(len--)
+        {
+            sum1 += *data++;
+            sum2 += sum1;
+        }
+        if(sum1 >= ADLER_MODULE) sum1 -= ADLER_MODULE;
+        sum2 %= ADLER_MODULE; /* only added so many ADLER_MODULE's */
+        ctx->sum1 = sum1 & 0xFFFF;
+        ctx->sum2 = sum2 & 0xFFFF;
+        return 0;
+    }
+
+    /* do length NMAX blocks -- requires just one modulo operation */
+    while(len >= NMAX)
+    {
+        len -= NMAX;
+        n = NMAX / 16; /* NMAX is divisible by 16 */
+        do {
+            sum1 += (data)[0];
+            sum2 += sum1;
+            sum1 += (data)[0 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8];
+            sum2 += sum1;
+            sum1 += (data)[8 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2 + 1];
+            sum2 += sum1;
+
+            /* 16 sums unrolled */
+            data += 16;
+        } while(--n);
+        sum1 %= ADLER_MODULE;
+        sum2 %= ADLER_MODULE;
+    }
+
+    /* do remaining bytes (less than NMAX, still just one modulo) */
+    if(len)
+    { /* avoid modulos if none remaining */
+        while(len >= 16)
+        {
+            len -= 16;
+            sum1 += (data)[0];
+            sum2 += sum1;
+            sum1 += (data)[0 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8];
+            sum2 += sum1;
+            sum1 += (data)[8 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2 + 1];
+            sum2 += sum1;
+
+            data += 16;
+        }
+        while(len--)
+        {
+            sum1 += *data++;
+            sum2 += sum1;
+        }
+        sum1 %= ADLER_MODULE;
+        sum2 %= ADLER_MODULE;
+    }
+
+    ctx->sum1 = sum1 & 0xFFFF;
+    ctx->sum2 = sum2 & 0xFFFF;
     return 0;
 }
 
diff --git a/adler32.h b/adler32.h
index 0761340..dc208a8 100644
--- a/adler32.h
+++ b/adler32.h
@@ -20,6 +20,8 @@
 #define AARU_CHECKSUMS_NATIVE_ADLER32_H
 
 #define ADLER_MODULE 65521
+/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(ADLER_MODULE-1) <= 2^32-1 */
+#define NMAX 5552
 
 typedef struct
 {
diff --git a/fletcher16.c b/fletcher16.c
index d675699..748f2a1 100644
--- a/fletcher16.c
+++ b/fletcher16.c
@@ -1,21 +1,29 @@
 /*
  * This file is part of the Aaru Data Preservation Suite.
  * Copyright (c) 2019-2021 Natalia Portillo.
- *
- * This library is free software; you can redistribute it and/or modify
- * it under the terms of the GNU Lesser General Public License as
- * published by the Free Software Foundation; either version 2.1 of the
- * License, or (at your option) any later version.
- *
- * This library is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- * Lesser General Public License for more details.
- *
- * You should have received a copy of the GNU Lesser General Public
- * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
 
+/* adler32.c -- compute the Adler-32 checksum of a data stream
+  Copyright (C) 1995-2011 Mark Adler
+
+ This software is provided 'as-is', without any express or implied
+ warranty.  In no event will the authors be held liable for any damages
+ arising from the use of this software.
+ Permission is granted to anyone to use this software for any purpose,
+ including commercial applications, and to alter it and redistribute it
+ freely, subject to the following restrictions:
+ 1. The origin of this software must not be misrepresented; you must not
+    claim that you wrote the original software. If you use this software
+    in a product, an acknowledgment in the product documentation would be
+    appreciated but is not required.
+ 2. Altered source versions must be plainly marked as such, and must not be
+    misrepresented as being the original software.
+ 3. This notice may not be removed or altered from any source distribution.
+
+ Jean-loup Gailly        Mark Adler
+ jloup@gzip.org          madler@alumni.caltech.edu
+*/
+
 #include <stdint.h>
 #include <stdlib.h>
 
@@ -38,15 +46,118 @@ AARU_EXPORT fletcher16_ctx* AARU_CALL fletcher16_init()
 
 AARU_EXPORT int AARU_CALL fletcher16_update(fletcher16_ctx* ctx, const uint8_t* data, uint32_t len)
 {
-    uint32_t i;
     if(!ctx || !data) return -1;
 
-    for(i = 0; i < len; i++)
+    uint32_t sum1 = ctx->sum1;
+    uint32_t sum2 = ctx->sum2;
+    unsigned n;
+
+    /* in case user likes doing a byte at a time, keep it fast */
+    if(len == 1)
     {
-        ctx->sum1 = (ctx->sum1 + data[i]) % FLETCHER16_MODULE;
-        ctx->sum2 = (ctx->sum2 + ctx->sum1) % FLETCHER16_MODULE;
+        sum1 += data[0];
+        if(sum1 >= FLETCHER16_MODULE) sum1 -= FLETCHER16_MODULE;
+        sum2 += sum1;
+        if(sum2 >= FLETCHER16_MODULE) sum2 -= FLETCHER16_MODULE;
+
+        ctx->sum1 = sum1 & 0xFF;
+        ctx->sum2 = sum2 & 0xFF;
+        return 0;
     }
 
+    /* in case short lengths are provided, keep it somewhat fast */
+    if(len < 11)
+    {
+        while(len--)
+        {
+            sum1 += *data++;
+            sum2 += sum1;
+        }
+        if(sum1 >= FLETCHER16_MODULE) sum1 -= FLETCHER16_MODULE;
+        sum2 %= FLETCHER16_MODULE; /* only added so many FLETCHER16_MODULE's */
+        ctx->sum1 = sum1 & 0xFF;
+        ctx->sum2 = sum2 & 0xFF;
+        return 0;
+    }
+
+    /* do length NMAX blocks -- requires just one modulo operation */
+    while(len >= NMAX)
+    {
+        len -= NMAX;
+        n = NMAX / 11; /* NMAX is divisible by 11 */
+        do {
+            sum1 += (data)[0];
+            sum2 += sum1;
+            sum1 += (data)[0 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8];
+            sum2 += sum1;
+            sum1 += (data)[8 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2];
+            sum2 += sum1;
+
+            /* 11 sums unrolled */
+            data += 11;
+        } while(--n);
+        sum1 %= FLETCHER16_MODULE;
+        sum2 %= FLETCHER16_MODULE;
+    }
+
+    /* do remaining bytes (less than NMAX, still just one modulo) */
+    if(len)
+    { /* avoid modulos if none remaining */
+        while(len >= 11)
+        {
+            len -= 11;
+            sum1 += (data)[0];
+            sum2 += sum1;
+            sum1 += (data)[0 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8];
+            sum2 += sum1;
+            sum1 += (data)[8 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2];
+            sum2 += sum1;
+
+            data += 11;
+        }
+        while(len--)
+        {
+            sum1 += *data++;
+            sum2 += sum1;
+        }
+        sum1 %= FLETCHER16_MODULE;
+        sum2 %= FLETCHER16_MODULE;
+    }
+
+    ctx->sum1 = sum1 & 0xFF;
+    ctx->sum2 = sum2 & 0xFF;
     return 0;
 }
 
diff --git a/fletcher16.h b/fletcher16.h
index 2aa2f58..8742895 100644
--- a/fletcher16.h
+++ b/fletcher16.h
@@ -20,6 +20,8 @@
 #define AARU_CHECKSUMS_NATIVE_FLETCHER16_H
 
 #define FLETCHER16_MODULE 0xFF
+/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(ADLER_MODULE-1) <= 2^16-1 */
+#define NMAX 22
 
 typedef struct
 {
diff --git a/fletcher32.c b/fletcher32.c
index 246bb6d..7323517 100644
--- a/fletcher32.c
+++ b/fletcher32.c
@@ -38,15 +38,138 @@ AARU_EXPORT fletcher32_ctx* AARU_CALL fletcher32_init()
 
 AARU_EXPORT int AARU_CALL fletcher32_update(fletcher32_ctx* ctx, const uint8_t* data, uint32_t len)
 {
-    uint32_t i;
     if(!ctx || !data) return -1;
 
-    for(i = 0; i < len; i++)
+    uint32_t sum1 = ctx->sum1;
+    uint32_t sum2 = ctx->sum2;
+    unsigned n;
+
+    /* in case user likes doing a byte at a time, keep it fast */
+    if(len == 1)
     {
-        ctx->sum1 = (ctx->sum1 + data[i]) % FLETCHER32_MODULE;
-        ctx->sum2 = (ctx->sum2 + ctx->sum1) % FLETCHER32_MODULE;
+        sum1 += data[0];
+        if(sum1 >= FLETCHER32_MODULE) sum1 -= FLETCHER32_MODULE;
+        sum2 += sum1;
+        if(sum2 >= FLETCHER32_MODULE) sum2 -= FLETCHER32_MODULE;
+
+        ctx->sum1 = sum1 & 0xFFFF;
+        ctx->sum2 = sum2 & 0xFFFF;
+        return 0;
     }
 
+    /* in case short lengths are provided, keep it somewhat fast */
+    if(len < 16)
+    {
+        while(len--)
+        {
+            sum1 += *data++;
+            sum2 += sum1;
+        }
+        if(sum1 >= FLETCHER32_MODULE) sum1 -= FLETCHER32_MODULE;
+        sum2 %= FLETCHER32_MODULE; /* only added so many FLETCHER32_MODULE's */
+        ctx->sum1 = sum1 & 0xFFFF;
+        ctx->sum2 = sum2 & 0xFFFF;
+        return 0;
+    }
+
+    /* do length NMAX blocks -- requires just one modulo operation */
+    while(len >= NMAX)
+    {
+        len -= NMAX;
+        n = NMAX / 16; /* NMAX is divisible by 16 */
+        do {
+            sum1 += (data)[0];
+            sum2 += sum1;
+            sum1 += (data)[0 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8];
+            sum2 += sum1;
+            sum1 += (data)[8 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2 + 1];
+            sum2 += sum1;
+
+            /* 16 sums unrolled */
+            data += 16;
+        } while(--n);
+        sum1 %= FLETCHER32_MODULE;
+        sum2 %= FLETCHER32_MODULE;
+    }
+
+    /* do remaining bytes (less than NMAX, still just one modulo) */
+    if(len)
+    { /* avoid modulos if none remaining */
+        while(len >= 16)
+        {
+            len -= 16;
+            sum1 += (data)[0];
+            sum2 += sum1;
+            sum1 += (data)[0 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[0 + 4 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8];
+            sum2 += sum1;
+            sum1 += (data)[8 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 2 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 1];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2];
+            sum2 += sum1;
+            sum1 += (data)[8 + 4 + 2 + 1];
+            sum2 += sum1;
+
+            data += 16;
+        }
+        while(len--)
+        {
+            sum1 += *data++;
+            sum2 += sum1;
+        }
+        sum1 %= FLETCHER32_MODULE;
+        sum2 %= FLETCHER32_MODULE;
+    }
+
+    ctx->sum1 = sum1 & 0xFFFF;
+    ctx->sum2 = sum2 & 0xFFFF;
     return 0;
 }
 
diff --git a/fletcher32.h b/fletcher32.h
index c5d991f..09b5156 100644
--- a/fletcher32.h
+++ b/fletcher32.h
@@ -20,6 +20,8 @@
 #define AARU_CHECKSUMS_NATIVE_FLETCHER32_H
 
 #define FLETCHER32_MODULE 0xFFFF
+/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(ADLER_MODULE-1) <= 2^32-1 */
+#define NMAX 5552
 
 typedef struct
 {