This is a relatively simple hint, but it makes sense for people who use X11 (XDarwin) in rootless mode. A shell environment variable "DISPLAY" tells XWindows applications where the windows should display. When you start XDarwin, DISPLAY is automatically set to :0.0, but this variable only gets exported to new xterm shells.
xterm is slow while scrolling, yet Terminal.app is quite quick. Why not use Terminal.app instead of xterm? To do this, use this command in a variant of csh.
setenv DISPLAY :0.0(if you don't use a variant of csh, you will need to know how to set environment variables in your shell.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011207142141772