I've encountered some slowdown with the Gumball Bonus Stage, the one with the electromagnetic spheres. This only happens though when there's too much going on, and having hundreds of rings. Happened more often when playing with Sonic & Tails.
I've also encountered some slowdown dropping hundreds of rings, but I think that part of it is normal.
I've played these games hundreds of times with the Mega EverDrive X7 and more recently the Mega Sg's JB. It just seemed a little bit slower than usual. I think some more testing could be done and just want to see if I'm not the only one having this problem because I can't tell anymore. I wanted to see what other people had to say. I do not have an original Mega Drive to test with anymore.