Animated margin-top doesnt have a fix speed

Animated margin-top doesnt have a fix speed

Hello guys, hope you can help me out.

Im animating a div to go up, its a long animation upto -8000px but it doesnt show a fix speed, at the begining is really slow and speeds up then at the end it slows down.
I would like it to have a fix speed, do you know any plugin or anything to help out?

Thanks!