In Ubuntu I use RadRails (actually the Aptana IDE with RadRails support) and I don't find it slow at all. My computer is nothing special: 2ghz cel, 512m ram. I don't use Radrails to run my webrick server and to tail files or to run tests or to generate files. Maybe that's why radraisl isn't slow for me. I find that the good 'ol console is best for those things anyways.
And that's exactly why I can't recommend this next program enough. Yakuake is an amazing tabbed terminal that slides down from the top of your screen with the press of a key (f12 by default). I open one tab to run autotest, one for a rails console, one for a development server (webrick/mongrel/lighttpd), one each for tails on the dev and test logs, and one for running commands (script/generate whatever, etc). Yakuake stays out of your way and is always there when you need it, at the same time. Often I need to switch quickly (with the keyboard) between radrails and firefox and it really helps that yakuake doesn't "show up" when you are tabbing between them. Yet it's always in front as soon as you need it. I can't say enough good things about this simple program.
It is an awesome setup for a mediocre system and will do me just fine until I can get a Mac. Sorry Eric. lol