Outlook 2010 is actually much improved here over previous versions, but it's hardly shocking that Outlook would be more demanding of resources, it's probably the most complete and full-featured email client on the planet.
Hello, woefully backwards HTML email rendering, lack of full RFC compliance (especially w.r.t. etiquette guidelines on message formatting), no support for tree-threaded conversations, and no provision for spoofing the from address (essential for when using disposable email addresses). These are things that Thunderbird has supported since it was released in 2004. I'm just saying.
(I'm an Outlook user myself, 2003 on my desktop, 2010 on my laptop).
You should NOT see the "work in progress" spin icon when just right-clicking on the desktop for pete's sake, even for a split second.
That's often the work of nasty shell extensions that insert useless and shameless advertising into the desktop context menu. I'm going to start pointing fingers at Intel and NVidia. When either are enabled I get a delay in opening the desktop context menu. Fortunately it's easy to fix by removing all traces of the shell extensions with Sysinternals Autoruns.
Not to tout OSX as the perfect OS by any means, it certainly isn't - but user-interface responsiveness in the face of high I/O, in my (admittedly limited) experience, it has Win7 beat handily.
Amen, brother. Unfortunately this won't be fixed, ever, due to various reasons I won't go into.