Tweaking REAPER for lowest Software Drumming latency 👉

For all plug-ins involved (including EZ3) (and including SD3, when latency is introduced by internal SD3 audio effects or by resampling): Open the "Plug-in pin connector", click "I/O", and disable “Plug-in delay compensation (PDC)”, as this always increases latency to the full audio block size (or a multiple of it).

Experiment with the “Behavior” settings based on your specific CPU and its configuration. (Preferences ➜ Audio ➜ Buffering).
(The "Behavior" parameter affects Reaper's CPU scheduling strategy. Higher values make Reaper more aggressive in distributing DSP tasks across cores and processing them ahead of time, while lower values use a more conservative scheduling approach.)

Especially if your VST-e-drum routing involves tracks (/ audio effects on those tracks) that are not part of Reaper's real-time engine: Globally deactivate "Anticipative FX processing" (Preferences ➜ Audio ➜ Buffering) or set "Prevent anticipative FX" on all relevant tracks under "Track performance options"

Otherwise, leave all settings under “Buffering” at their default values for now.