I currently use Ubuntu 10.10, and I find the emulator extremely slow. However, I have an Intel Core 2 Duo CPU (T5200 processor) with 4G of RAM. What do I need to improve performance? Better hardware? A minimal Linux distribution?
Past versions of the emulator were extremely slow on all platforms. However, there are now native x86 versions of the emulator; performance should be considerably improved. You should be able to get them through the Android SDK (or through Intel's website, see this Stack Overflow post).
As an update to Matthews answer: I'm running Android-x86 in a VirtualBox VM, performance and load are pretty much acceptable. Android-x86 images are available for download on the project site, and install in VM works like with any Linux distro (basically).
for detailed instructions, there are several tutorials around; simply google for android-x86 virtualbox, good matches appear already on the first page, e.g.