I have no clue on whether the setting in game works or not, but I've never felt the game turning/looking/etc slow, Perhaps you've got a multi-setting mouse with a button on it, or perhaps your desktop is set to have the mouse slower? if you right click desktop, personalize, and click for changing mouse pointer, you can click pointer options after the new window opens and adjust mouse speed.
It's also possible (maybe) that you've additional brand software for your mouse that hogs authority and keeps it a certain amount, and that amount happens to translate poorly for MLBS, while other games may have insanely high settings that most would've lowered, While MLBS is just normal, but it's possible the ingame slider still doesn't work at that point (again, I dunno about that part, I'm more focused on the fact that its slow, and trying to work on that point)