I can report a *tiny* amount (negligable) of scroll slow down if I enable cleartype on my desktop PC and switch to courier font. However, I have not tested this with a huge amount of text in my scroll-back buffer.
I would expect this to happen, though, as the computer has to anti-alias the text on-the-fly at a very quick rate as you scroll up or down. This is, I imagine, quite a CPU intensive task to perform.
My system stats for your comparison:
XP Pro SP1a
Athlon XP 2400+ 2 GHz processor
512 MB DDR ram
Radeon 9200 SE gfx card
