the mod works just fine, but when I alter or delete it, then my speed is lower, like much lower

Just had this problem in 0.18

I fixed it by enabling the mod, then saving while running at full speed, then disabling the mod and loading that save.

I think the mod works by setting the run speed of the player, so when it's disabled on a save the run speed is locked to that low value.

See "how to properly remove" on the info page. I'm sorry about this, but once a mod is removed, it can't change the game any more, even to fix stuff it left behind.