It isn't accurrate as far as I have observed. IE is pretty slow, but try Mozilla, its lightning fast. I think the slowness should be judged on an application by application basis.
I do think optimization is sorely needed in the Cocoa (and to a lesser extent, Carbon) APIs. For Carbon, I think application fine tuning can yield great results. Cocoa seems to lag a bit in all applications and I think that behavior can be improved.
PCs being twice as fast is a bit of an exaggeration from my experience. I run a 1.4 GHz P4 at work and it renders pages somewhat faster than my 400 Mhz G4 TiBook, but its not outstandingly fast or even noticeable. Compared to my G4 400 Tower with a Radeon AGP graphics card, its reasonably similar.
Now keep in mind that OmniWeb is a bit slower than Mozilla. But it gives me features I crave so I take the slight speed hit. IE in windows isn't all that fast and it doesn't look as good or hav some of the features that OmniWeb has (spell checking, download manager, etc.)
Agreed that OW has gotten much faster. Try the latest Mozilla, though. They're still not even close, especially for rendering tables (which Gecko just rocks at) or complex pages. Maybe it has something to do with OW using Quartz AA. OW gets points for being Cocoa, though.