Not sure if this issue here is related to Steve's but it's really severe and also affects the MIDI editor, so I guess it is. I made a video to show the whole impact.
Basically the MIDI editor gets almost unusable over time. More about it in the video:
http://nextcloud.stephanroemer.net/i...aeiZmiAfC4n3pX
Hence, no other parts of Reaper are affected. Everything else works without any speed loss.