I am still testing when this comes up, however, when I play in 2 player mode, I notice it stutters a little bit, intermittently while playing. Most of the songs I have all have a video with it, so I will be trying some some without videos to verify if its part of the issue or not. So not the entire time, but comes and goes. When I play just 1 player mode, via keyboard or dancepad (using a usb arcade encoder in the pad) it is much much better. Not 100% smooth, but enough to where it does not mess with me. I tried with 1 pad and a keyboard and 2 pads and the issue persists. I normally see it running around 500+ FPS if vsync is off, and I run it in 4K. Does not matter if I turn it down to 1080p, nor nothing much else that I have tried. I have used a loading program as well in the background to load the process up as well (though it was set to below normal for its process affinity).
I have tried Stepmania 5.0.12 as well, which is mostly smooth, like if I played just single player on 5.3 Alpha 4.8.2.