diff --git a/dist-build/msys2-win32.sh b/dist-build/msys2-win32.sh
index a1e73fd8be..dcc9ee06a0 100755
--- a/dist-build/msys2-win32.sh
+++ b/dist-build/msys2-win32.sh
@@ -1,6 +1,6 @@
 #! /bin/sh
 
-export CFLAGS="-O3 -fomit-frame-pointer -m32 -march=pentium3 -mtune=westmere"
+export CFLAGS="-Ofast -fomit-frame-pointer -m32 -march=pentium3 -mtune=westmere"
 export PREFIX="$(pwd)/libsodium-win32"
 
 if (i686-w64-mingw32-gcc --version > /dev/null 2>&1) then
diff --git a/dist-build/msys2-win64.sh b/dist-build/msys2-win64.sh
index 2a7961a800..0b38d6dd28 100755
--- a/dist-build/msys2-win64.sh
+++ b/dist-build/msys2-win64.sh
@@ -1,6 +1,6 @@
 #! /bin/sh
 
-export CFLAGS="-O3 -fomit-frame-pointer -m64 -mtune=westmere"
+export CFLAGS="-Ofast -fomit-frame-pointer -m64 -mtune=westmere"
 export PREFIX="$(pwd)/libsodium-win64"
 
 if (x86_64-w64-mingw32-gcc --version > /dev/null 2>&1) then