Yea you want low speed on your mouse for FPS games, what I mean by that you move your mouse a lot for not much movement on screen.
It depends on the game of course, but I play PUBG, lower speed is better for fine aim, a lot of PUBG is at range, and if you are aware of your surroundings which as an experienced player you should be, you will already be facing more or less your target, which then leaves the fine aiming, particularly with a scope.
Even close range, again you should be aware of the positions of players so shouldn't ever have to be spinning round doing 180's etc.