I looked at that video of Sonic Genesis, and yes, it does look like it's got some pretty bad problems with slow-down. Some of the weirdos in the comments box tried to claim that there was no slowdown - that it was the video on YouTube! Hah! When converting 60 FPS video to 30 FPS video, there's no loss in pace! I told them to look at the flashing RINGS counter. Maybe that'll show them something.
Strange thing is, though, that the Sonic Advance games ran at full-speed. Guess it's just a shoddy port.