FreeFileSync Forum

Ah, so it's a CPU not file I/O bottleneck. Maybe this is what has been reported here:
[404, Invalid URL: https://sourceforge.net/p/freefilesync/bugs/223/]
Alas it seems I scared the original poster off by a snarky remark.
Anyway, I can create a new test version if you help to debug this problem ...

I'm experiencing the exact same thing. Even after just doing a Compare, after some time, Windows XP Task Manager will show FreeFileSync_Win32.exe using 99% of CPU. Had to revert back to 5.9.