creating msdosfs (fat32) locks the system

[[LoN]Kamikaze]

I'm running RELENG_7-BETA2. Creating files (e.g. by copying something) to a
fat32 mount locks up the whole system for a moment. This is best witnessed
when copying many files. With slow files the mouse will start to feel jaggy
and music plays slower.
When creating large files, you'll actually hear the music freeze (which sounds
quite nasty) and see that the whole X session freezes (i.e. no mouse movement
and the screen is entirely frozen).

I have this problem ever since I switched from RELENG_6 to RELENG_7. I didn't
find a PR about it, should I create one?