I am not sure why the ATI issue is still so prevalent- in essence, they have a driver installer that works very well. The distro binaries are hit and miss, but, running the installer outside of x, at least in debian and debian based distros, seems to work fine, with relatively decent performance.
That having been said, while my experience with Unity is limited to trying it for shits and giggles, Gnome3 does appear to use resources enough to slow perceived performance. On otherwise identical laptops, firegl card, one with two gig and the other four of ram, there is a noticeable difference in "feel". Obviously, 3d desktops will consume more resources than 2d, but at 6 gig of RAM, sing the proprietary driver, response in a properly installed system should be at least windows equivalent.
However, that installation is as automatic as the Ubuntu/Mint people would have us believe is at best putative. After install, updates and drivers make a huge difference. Optimised 3d hardware acceleration out of the box with the latest card and open source drivers?
All good wishes,
Life is a series of return dates. There is but one final argument, its eloquence determines who we were, and whether who we were had meaning.
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RAM is hardly a driver of performance. Unity doesn't consume much RAM.
My laptop is very slow under both Gnome3 and Unity. It is a 32-bit system with 4-gigs of RAM. When I boot up ~300M is in use.
I'm not sure where the performance problem is coming from, but it is not a RAM issue.