Looks like the consensus is that it's noticeably different to lion. I guess every new OS demands more
I'm not sure that that's strictly true; newer OSes should be more efficient on the whole as the trend is towards faster startups of the OS and its apps, as well as better battery life, so all sorts of things are being optimised continually.
The main area OSes have creeped up has been RAM, but Lion seems to have started a trend at Apple towards modularising more of the OS features into their own discreet processes, which means they can be kept closed until needed, so that saves on RAM and maybe some CPU performance.
Otherwise the main area is GPU use, as OSes are still doing fancier things visually, which is generally a good thing if it leads to greater usability, but means more GPU time and VRAM spent on the OS.
In Mountain Lion this seems to include some graphics enhancements that help to make things smoother similar to what Quartz etc. did in the past, presumably to make it even more friendly for touching and dragging. So things certainly seem smoother, more responsive, but in general I think things aren't
much faster in practise, just a bit easier to use.