It obviously slows you down a little, but not enough to matter.
I love my case. It protects the camera, looks great, and best of all improves the handling with the little grip that is on the front. The neck strap is comfortable, too.
On my M5 I just unsnap the one side leaving the strap still attached to the camera and slide the camera out of the case. You change the film, reinsert the camera and then resnap the case through the one strap.
On my MP3, you just unsnap two snaps which go over top of the eyelets. Remove the camera from the case with the stap still attached, and change your film. Reinsert you camera, snap the two little straps over the eyelets and you are done.
Best,
Ray