I always thought your computer slowed down when copying between partitions because you are both reading and writing on the same drive while running the OS. Although with 7 so far I have never experienced slowing when multitasking. It's actually pretty awesome.
I've tried both. Copy from partitions of the same drive as well as form external hdd. Of course with two different drives it's better, but in both cases even Vista was faster than Win7. But that is not the real problem. I could live with a sluggish copy process. But I can NOT live with a nearly freezing system as soon as i start any copying. Something is very wrong here - despite the fresh Win7 installation.