I had performance problems between 2018.2 and 2018.3 b12 on some UI panels. The more complex the panel the worse the framerate. The cause was having 2 'Canvas Scaler' components on the Canvas. One was the default 'Constant Pixel Size' canvas scaler, the other was 'Scale With Screen Size'. Having both at the same time caused the frame rate to go from 200fps in 2018.2 to 100fps in 2018.3. I do not consider this a Unity bug, I should not have had 2 Canvas Scalers on the Canvas. But since it didn't affect performance in 2018.2 I never noticed, and copy pasted that canvas around to a few other places. Removing the 2nd unwanted Canvas Scaler fixed the fps problem.