There is a solution but it seems that it works only once - the first run (adding to a shortcut LD_PRELOAD=$HOME/linusmemcpy.so /usr/bin/firefox &) . Then an error occurs:

Failed to execute child process "LD_PRELOAD=$HOME/linusmemcpy.so" (No such file or directory)

linusmemcpy.so exists in the location but it does not work.

It was by the advice:
1. Cut and paste this into a prompt:
---Cut below---

cat > $HOME/Downloads/linusmemcpy.c <<EOF
#include <sys/types.h>

void *memcpy(void *dst, const void *src, size_t size)
{
void *orig = dst;
asm volatile("rep ; movsq"
:"=D" (dst), "=S" (src)
:"0" (dst), "1" (src), "c" (size >> 3)
:"memory");
asm volatile("rep ; movsb"
:"=D" (dst), "=S" (src)
:"0" (dst), "1" (src), "c" (size & 7)
:"memory");
return orig;
}
EOF
cd $HOME/Downloads
gcc -O2 -c linusmemcpy.c
ld -G linusmemcpy.o -o linusmemcpy.so

LD_PRELOAD=$HOME/Downloads/linusmemcpy.so /usr/bin/firefox &

LD_PRELOAD=$HOME/Downloads/linusmemcpy.so /opt/google/chrome/google-chrome &