I would say it is quite linear with the size of your synced folders (and the amount of files, a lot of small files need longer than a few big ones). I have almost 100GB on a pi 1 and it works, never saw any deadlocks due to it being too slow, only some when there was a real bug 
If you add a lot of stuff at once you may run out of RAM (at least someone did here Huge RAM usage (2GB+) on synology DS1513+) but otherwise I don’t see any problems.