I'm getting ~60 fps when there are only a couple of HTML elements on the document. The rendering time looks fine (actually it's quite impressive!) but the CPU takes ~15ms per frame. I wonder if the FPS is locked internally in PowerUI?
EDIT: after some deep profiling, I discovered that the framerate is actually locked in PowerUI (and I found this article about UI.SetRate).
In the Unity profiler, I also discovered that PowerUI consumes a lot of memory. In my test, there were 100 moving texts and 100 flickering images, and the memory usage was about 10x more than the test using UGUI. I understand HTML rendering is a heavy thing but I wonder if any plan for the memory optimization?