Because you can paste between Adobe Illustrator CC and Adobe Animate CC so easily. You are ..lets say too easy. When you paste a complex gradient (And you can create, really very very complex gradients in Adobe Illustrator) these get - during pasting into Adobe Animate - converted to SVG.
But as every old school postscript guy knows these files or SVG have thousands, maybe millions of control points. That was a little too much....It got even the ventilator spinning of my Macbook
Maybe that works in print, but for online in a banner ...i just ditched the gradients made it one solid color (No one will notice) and voila...every thing running smooth.
mstrhans's post in A first in GSAP, jerky, stuttering playback in GSAP? was marked as the answer