Testing performance under Vista can be an interesting experience. The trouble is, Vista tries to improve its performance by observing what you do and caching it for later use in case you do it again. This leads to something akin to the observer effect in science, where the very act of observing something has an effect on what it is you are observing.
Every time I test Ribbon tab switching performance in Vista, the results improve. In XP, the worst tab switching time I saw on my Core2 PC was 1.6 seconds for the first exposure of the Tools tab. In Vista, the same thing took 1.2 seconds the first time I tried it, but subsequent attempts (after closing and restarting AutoCAD but not rebooting) gave results in the 0.5 to 0.6 second range.
I tried turning off Vista’s pretty Aero interface and saw improvements of about 0.1 seconds in every measurement. With Aero off and testing a second time, I saw first tab switches from 0.2 to 0.6 seconds and second tab switches between 0.3 and 0.5 seconds. Most switches were around the 0.3 second mark.
While I hesitate to base any firm conclusions on such shaky ground as these rather unscientific tests, I can say this with some confidence: it is possible to have a system running AutoCAD 2009 under Vista where the Ribbon tab switching performance meets in most cases, and is close to meeting in all cases, the 0.3 second mark that most users perceive as instant response. However, your mileage may vary. Correction: your mileage will vary.