Let's stop discounting the issue please.
There is a certain type of authority on messageboards like these who is very quick to dismiss perceived issues of software performance, immediately chalking it up to unrealistic expectations of the thread author, or the inadequacy of their hardware. And truly, those are always possibilities.
But while we now know that the doubts about the author's CPU were unfounded, I've got a crazy idea for you guys.
How about just giving the benefit of the doubt to the author, eh? Consider: Anyone who bothers to login and post an issue like this, who is then big-upped by a chorus of other people who ALSO bothered to Google this thread and respond, might actually know what they're talking about.
As a Mac user from 1985, a user of Mac design software since 1986, from Pagemaker through Quark to all iterations of InDesign, and as the owner of a 2.8ghz Core 2 Duo iMac with 4gb of ram (plenty!) I am here to say that this version of InDesign CS5 lags about like QuarkXPress from the late-1990s did on machines that were a thousand times slower.
I remember a few years ago, a bunch of messageboard "authorities" telling other Mac users who had problems playing low-def Flash video on very capable G5 computers, that their computers were incorrectly configured, had inadequate video cards, all kinds of nonsense. It turns out Adobe just didn't want to pay anyone to write a Flash player that worked on the Mac PowerPC architecture. The Mac Flash player for PowerPC was, and is, some ridiculously compiled cheap-arse non-native code that used 100% of CPU power and overheated the machine.
Sometimes, the software is just badly written, and not only in someone's imagination. InDesign CS5, for users I've talked to who depend on this kind of software professionally and know it well, is just noticeably laggy on redraw/resize. Not with hi-res images, not with complicated layerings - it is just plain slow on ordinary performance. Maybe not to you, casual user. But I notice it, and plenty of other people notice it.
Some people just like to apologize for large organizations like Adobe, Apple, whoever. But folks, organizations also do lazy things sometimes. And let's not forget - companies as big as Adobe actually hire PR people to go on messageboards like this, and downplay the problems with their products.
Please, if you don't know why it lags, or maybe why Adobe didn't rewrite InDesign for Mac properly this time, spare us the "Maybe it's you" answers. And thanks to everyone else for posting their experiences.
Macstudent - turning off "immediate" redraw did not help for me. Thanks for the suggestion though.