Weird one this, installed Waves Synth plugin Element and it all went foobar.
First off I couldn’t find it in Reaper as it doesn’t install as a VSTi instead it installs as a VST effect.
When I did get it running it was dog slow.
So I decided to uninstall and do a fresh install.
Clear cache/Rescan and it seems better but still sluggish.
Open Task Manager and find the culprit – Reaper Host 32 is hogging a lot of processor power.
End that task and everything is working fine.