On my iPad Pro 12.9" 2nd gen, the preview thumbnail for the current app now animates smoothly in the multitasking screen in 11.3.
Starting in the late 11.0.0 betas they broke something that made the current app preview animate its content at 5-10 fps, which was very apparent when activating multitasking while watching a video. The bug kicked in as soon as it would start the transition from full screen to thumbnail, and would only affect the content, the size transition itself was perfectly smooth.
I'm glad they fixed this at last in 11.3, it looked a little jarring on my "high end" iPad which otherwise is very fast.