Be it as it may, but xterm/X11 still kills Terminal.app in text rendering speed. Also, MacVim is, on my Mac at least, visibly slower in text rendering than the naked vim under xterm.
I'm assuming this isn't so slow it shows up on a screen recording (of course, if it IS, that would be interesting), but either way could you please file a bug at http://bugreport.apple.com/? It will reach someone who cares.
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)
