If you go into the game files and play the animations in a video player, it runs fine, right?
But when you run it it in renpy and open the task manager, you can see it's capping out your CPU usage, right?
Yea, this happens to me as well. Renpy sucks at playing animations. I've tried forcing it to run on my dedicated graphics card instead of the integrated one. I've tried Shift + G to mess with the renderer and FPS caps. I've tried playng the conpressed version of the games.
Not all renpy games have laggy animations, but for those that do, nothing I did fixed the issue.
Maybe someone can figure out a solution one day.