Only the vocals are processed in Reaper. The guitar goes to FOH from the Kemper and the drums are sent into the PA. The click track also comes out of the interface into headsets for the drummer (probably going to replace with in-ear monitoring); and then the vocals are sent out audio interface to the PA. Reaper is mainly used for MIDI patch changing, full control of the vocalist effects, and any overlays that are necessary.
Have not experienced any latency issues. I guess to compare, think of when you record guitar using Reaper. Are you hearing any latency from what you are playing and the alignment on the DAW? I know some do, but I have an average laptop, that has been gutted and thinned out of any extras to run just the software. So I have had no issues with latency.