Performance note on the current state of affairs, if you're using latency compensated plugins across multiple channels and instances on a track, is this report here that I posted a few days ago
https://forum.cockos.com/showthread.php?t=208670
It's likely just the way things are at the moment. Multi-mono/stereo instances to the rescue. Only one PDC value to consider. I say this because when you put up three identical(AFAIK) instances of ReaGate with identical, numerically entered pre-open values, Reaper actually adds the latency compensation values foe each plugin together, even though they're not processing the same channels. They do READ from the same set(ch7/8), but each of the three stereo instances processes two different channels.
Reaper has a lot of dropouts for the first few seconds of playback, less so when there's no audio coming in through the sidechain.
So that's a little fucked
.