I have two external effects pedals assigned to the FX Loop effect. Both are Eventide pedals (Timefactor & Modfactor). I run the pedals mono, through their left-mono outputs. These are digital pedals, not analog.
I hear obvious latency when engaging either / both pedals. It sounds like a slap echo, and a bit hollow. Not bad if that's what you want, but....
I have successfully run these two pedals through FX loops, also in mono, on analog amps with no audible latency.
I have tried the FX Loop in Pre and Post, and placing it both before and after the Stack section in the signal flow. While I do hear some audible differences, the latency remains. I can dial it out a little by reducing the FX Loop volume a bit, but that's not a perfect solution.
I'm thinking that this may be the inevitable result of both the external pedals and the Kemper doing their A/D to D/A conversions, serially, and that the latency adds up. I'm not exactly sure how the FX Loop is routed logically within the Kemper. In analog amps it's usually an insert between the preamp section and the power section (if I understand it correctly) but that may not be the case with the Kemper, or even really matter.