Just to add to this thread, got a heads up that an I’m having could be somewhat related: getting constant hitches when enabling motion blur on mesh particles, video here for context: UE4 Mesh Particle Motion Blur Hitch - YouTube
Also can confirm that closing the editor and reopening seems to reset the temporarily, but the longer it is open, the more obvious the gets.
On freshly opening the editor, I have constant stable 120FPS with a particle system running with thousands of mesh pieces. Then even 10 minutes later, the becomes so bad that same particle system is dipping from 120 down to 10FPS or lower with extremely obvious hitches lasting for what seems like almost a second of half second each.
Hopefully a fix is on the way!