[2.6] system is very slow during disk access

I have massive problems with linux-2.6.0-test2. When some process writes something to disk, it's very hard to go on working with the system.

Some test-szenario:

$ dd if=/dev/zero of=./test.file 

While dd is running, xmms skips playing every now and then and the mouse is near to be unusable. The Mouse-cursor behaves some kind of very lazy and some times it jumps from one point on the display to another.
When I stop disk-access, it works again quite fine.

Would be cool, if you could give me some point to start for tracking this down.