I use a Focusrite here, Scarlett 2i4, and also use Reaper.

I have no latency when recording guitar parts, but I also record them dry, i.e. no FX on the track. I add the FX after the track has been recorded. I think, but don't know for sure, if I attempted to record and also add FX at the same time I would get latency, could that be your problem?

I also monitor using the headphone jack on my 2i4.