This is odd, and I don't think I've ever seen it happen with any other VSTs.
If I have the GUI open (in either ADLplug or OPNplug), whenever I play a note, the meters on the mixer and track control panel freeze until I release the note.
https://github.com/jpcima/ADLplug
I'm using the latest Win64 binary from here.
Is this a thing? I've used a lot of VSTs in Reaper and I don't recall anything like this before. And there are no hardware acceleration options to tweak so I'm not really sure what to try.