The very first thing that annoyed me about the Apple was its default terminal. It's crap, it really is, its the slowest terminal I have used in my life, it was as bad as using a modem again.
The problem is of course that it tries to render everything in amazing quality and do transparency and use true type fonts and all that, it pretty much made for a completely unusable experience for me.
I tried a couple of alternatives like iTerm and while feature wise they are better they were still slow.
Another MAJOR pain is the fact that unlike Unix terminals the OS X native one does not copy text when you select it, I am just too used to that feature to not have it - there may well be a button to select somewhere to enable it, but I have not found it. Regardless the default terminal is too slow.
So I came across a shareware tool called GLTerm. It uses OpenGL to do it's rendering, at first this sounds like a stupid idea but it works, its really quick. Not as quick as Putty on my windows box but its fast enough and it supports copy on select which makes me happy, and $10 poorer.
UPDATE: The terminal in OS X is indeed much faster as pointed out below, it flies even on my old 800mhz ibook it still lacks copy-on-select though. Also you need to be aware that support from the GLTerm author is virtually non existent, you need to almost threaten the guy to get anything out of him, pity, it's a good terminal.
Apparently Terminal.app is going to be GL-accelerated in Tiger, so this may be a moot point :)
On the new PowerBooks at least, it doesn't feel slow at all. I still miss tabbed terminals though, that feature of gnome-terminal really rocks.