both MGS2 and MGS3 on the PS2 has framerate issues.
Oh no, let's not go there.
MGS2 had an incredible framerate on PS2 holding 60 fps the 99% of the time. Bouts of slowdown can occur in areas saturated with particles, but in general, it holds its 60 fps target very well. It's an incredibly smooth game.
MGS3, on the other hand, generally ran at 30 fps. Cutscenes could chop up a bit in some of the more demanding scenes (not present in this 3DS demo) but it managed to hold 30 fps during gameplay most of the time. Some of the more demanding scenes dropped but it wasn't that significant.
The 3DS version runs MUCH slower overall and features a lower level of detail on top of that. The average framerate on 3DS appears to be 20 fps rather than 30 (though this demo is smoother than some of the older footage that had been floating around). I'm also surprised at the reduction in texture quality. The 3DS has MUCH more memory than the PS2 did so I can't figure that one out.