Running any kind of build process would be an easy first example. :-)

Builds. sbcl in particular is insanely verbose and is often limited by slow terminals. The readme for bootstrapping it even specifically suggests using xterm over gnome-teriminal/konsole.

I get slowdowns caused by text rendering in terminal when rsyncing large numbers of files. It's easily worked around though, just minimize the terminal window

Try scrolling large amounts of text in MacVim, where the whole screen is refreshed, and then try the same in xterm+vim. MacVim is painfull to watch doing this, feel like working over a 300 baud modem. xterm/X11+vim are VERY fast combination for pure text editing, and that matters to me greatly. Same applies to Terminal.app (and it refuses to work in 256 colors, unlike xterm)