This is just an anecdote, but I share the parent's experience, too. I bought a low-end HP laptop around when Vista came out (2006/07?). It ran with Aero Glass, but not very smoothly. But first compiz-quinn and then beryl worked perfectly, also with the same blurred transparency effect that was slow on Windows. Even when I enabled the crazier effects like wobbly windows, it remained smooth, and used not noticably more resources. The only place a slowness was noticable was when quickly scrolling (webpages, for instance). However, this is still a problem on any non-Windows compositing DE in my experience.
At some point, "they" started to rewrite compiz, and it went downhill, until it was so slow I could not use compositing anymore.
On the Windows side, there was a lot of tense discussion between Intel and Microsoft regarding Intel's popular integrated-graphics chipset being shipped at the time of Vista's release. The chipset didn't have hardware DirectX support and shouldn't have been certified, but Intel had promised their customers (like HP) the chipsets were fine and they were not anywhere near the end of the product lifecycles for either the chipset or the PCs. So Microsoft fudged the rules and a bunch of consumer-grade hardware ran like crap on Vista.
At some point, "they" started to rewrite compiz, and it went downhill, until it was so slow I could not use compositing anymore.