Van, from Microsoft blogged about Whidbey performance, so here is my experience in response to his query:

I have a Pentium M 1.8 Ghz with 1Gb RAM.

I open the IDE and open a 1MB web solution, and the IDE is consuming 128.5 MB of RAM.  I don't feel any hurt because I have plenty more, but when i close the solution, I get the hour-glass, and there is a noticeable delay.  Also, when I create a new class file in the Code directory, I see a noticeable delay before the IDE is responsive again.  Then, if I add a class library project to the solution (to reference from my web site), every build is noticeable slower than VS 2003.  The IDE freezes for a moment before the build begins.  There are some intermitten freezes that happen, and since I'm a developer, even 1 second is noticeable.

Since you asked, I've used the MS IDEs all the way back to Visual Notepad.

On a positive note, the intellisense menus (which are EVERYWHERE) are super fast and responsive.  Kudos to the intellisense team!