Movement speed is a funny thing, and has a near total impact on the game. It effects almost everything.
From balance to map design, and the problem is, there isn't an easy solution. Bump up speed, and you have have performance issues (even if it doesn't cause issues for the server, higher speeds mean people can often "lag out"), combat issues (hit and runs are insane, melee fights are just silly and people trying to run can do it very well.)
Ect ect, it really is amazing how much of the game changes when you change movement speed.
Really deserves a bigger write-up but I am tired.