This speed thing does my head in.
I used ID1.5 throughout it's lifetime and yes it was sluggish sometimes, but offered superior productivity than that of Quark.
ID2 is much faster, and actually is closer to Quark than ID1.5 in both speed and function.
ID will always be slower than Quark, just as it will be always be slower than Pagemaker, not because of sloppy code etc but because it is far superior, has a modular (plugin like) program design and has to do much more at any one time than Quark. Take for example the Optical Kerning and Paragraph Composer, I marvel every time I use this function. I have applied Optical Kerning (reads and adjusts kerning based on how close the characters are to each other) to 40 pages of text before today and only had to wait a few seconds to see the difference.
Id sooner scroll slower down a very well drawn page that closely resembles to actable print than scroll fast down an old outdated and poorly drawn page.
Working in a pre-press environment as well, I can tell you I loath any time I spend in Quark nowadays.
Anyway, back to subject, I have used ID in OSX and its fine, although I found I had to install for both OS9 and OSX separately. The GUI certainly looks more tuned in OSX, in OS9 it sometimes looks and behaved worse than ID1.5, but I suspect that has something to do with it being Carbonised.