I did do some tests... The slowdown only happens in Chrome.
I made a very simple test case of the GSAP animation and it was perfectly smooth, but on my real case it isn't. The difference is that in the real case there are more layers of parent / ancestor elements for the target element.
The element was relatively positioned. Changing it to an absolutely positioned element made the effect much smoother... I think that in Chrome, there are some superfluous miniscule repositioning calculations being done, and maybe constant reflows. Chrome tends to do more anti-aliasing than Firefox (and thus reflows more, I guess).
I tried disabling requestAnimationFrame as well as replacing the tween with code from two other animation libraries, and the smoothness difference on relative versus absolute positioning were the same.
