It's been a while since I've felt the Ubuntu desktop was unresponsive. Ubuntu 11.04 is the first release in a while that's slow. In the past, it was always due to lack of RAM. I had 512MB of memory which was borderline. When there's not enough resources, the system slows down but is generally still usuable if you're willing to wait a little. A simple memory upgrade (I have 2.5GB now) fixed that.
Yet, Ubuntu 11.04 beats even that experience.
In particular when there's a lot of I/O activity, the desktop tends to "freeze". The system's still running -- there's hard disk activity -- but everything is slow as molasses. You can't even move the mouse pointer. Depending on the app that currently has focus, you may not even get to use Alt+SysRq+REISUB to restart your system safely.
Clearly, I'm not the only one experiencing this. That's why some of the improvements planned for Oneiric I mentioned in a previous post are needed. Anything that reduces resource consumption, or speeds things up is good. LightDM will help somewhat, although I think there needs to be a lot of work on improving Unity's overall responsiveness.