I was listening to the Podcast 17 interview of the GES team.
Emmanuel was saying that the game itself was lagging and the mouse movement was glitching.
He said, "Am I the only person who is getting this!?!?!"

He is not alone. I am getting the same problem.
Beta 3 ran smoothly for me, even when I had a few windows open.
Beta 4 is running horribly. I swear, it isn't lag. If I create my own server, the same thing happens and I don't know why.

I don't have a problem with my mouse. It runs perfectly outside of GES.

I have no idea what is going on.

Maybe try lowering your graphics settings in the main menu if you haven't done that already Emilia.

Options>Video>Advanced

I know beta 4 is a bit more demanding on one's resources than beta 3 was, and I get lower frame rates myself now which can cause that laggy/choppy feel.

I get this problem quite a lot, but not constantly, just when it feels like not working . Lowering the settings makes no difference, for me anyway; my computer is too good