Much slower than Compiz on an old i5 integrated GPU
So back in the day I reported this bug: https://bugzilla.gnome.org/show_bug.cgi?id=652097. So when I switched to overview it was choppy and only after I've interacted with the controls enough to force the GPU out of its power saving mode it got smooth. The bug got a NOTABUG response:
And regarding your question ... locking at the clock speeds above it seems that the memory bandwidth is way to low in "power saving mode" which results into your performance issues.
Although Compiz was smooth like butter on the same machine, I dismissed the bug...
But recently I decided to test various compositors on an about 7 year old machine using only its i5 CPU graphics, specifically how smooth is the Overview functionality. Here are the results:
GNOME Shell is extremely choppy, borderline unusable.
KWin is pretty smooth, maybe dropping a few frames here and there.
Compiz is perfectly smooth.
To me it seems like something is fundamentally wrong with how GNOME Shell uses OpenGL, JS doesn't seem like it's the culprit since I used GNOME Shell on the same machine but with an NVIDIA GPU and it worked well.