I'm on Win10. Indeed, I/O activity on the hd where Synology Drive is performing sync tasks can reach top levels (even spikes of 100%) but the c: drive where perl, firefox, and my scripts reside has been left out of Synology Drive's reach (I have another program that backs it up when my computer is idle). I guess it might bottleneck at the controller level but would that be so persistent as to cause problems even when the synchronization tasks are paused? There seems to be no I/O activity of this sort at the time. Yet, here we are.
Unless someone has a better idea, it may make sense to capture execution profiles of runs both with and without Synology Drive active, both Perl and C sides of things. The latter might demonstrate whether Synology Drive is doing something unusual, like injecting its DLL into other processes.