Gnome shell is CPU hungry and I can tell I'm not alone with that because I did a lot of google search on it. Even when dragging windows around, CPU usage skyrockets to 40-60% and it's not even smooth. And that's with gpu acceleration, I can't even imagine what would've happened if it were on software rendering. Not to mention memory usage of gnome shell only increases with time.
They are all known issues but shell developers blame proprietary Nvidia driver, maybe that's why you are more happy with gnome shell than I am :) (Though I still can't get how cpu usage skyrockets when dragging windows.)
I'm using Gnome shell and just tested what happens to the CPU usage when I drag around windows. The dragging itself was buttery smooth and gnome-shell never used more than 12.6% of one core. I'm using the libre radeonsi driver.
Totally unscientific: On my ancient Thinkpad T61p with nvidia graphics of some vintage and the proprietary drivers installed on Debian Jessie, I get around 12 to 20% on one of the two cores with vigourous mouse assisted window movement of an Iceweasel window. MATE desktop, compiz is not available in Jessie repos so using marco.
The overall user experience is smooth and pleasant. I doubt if Windows 8/10 would install on this machine (although it came with Windows 7). I have several Thinkpads of this vintage for the keyboards, not needing super powerful processors.
It's because of either different drivers, or your one core is faster than my poor little c2d core, or you're not moving it crazy enough :) Did you compare it with kwin? For example, on my machine gnome shell uses sth. like 40-60% while kwin is 10% at most.
They are all known issues but shell developers blame proprietary Nvidia driver, maybe that's why you are more happy with gnome shell than I am :) (Though I still can't get how cpu usage skyrockets when dragging windows.)