UMG renders stuff to texture in separate pass that is later used as an input texture in main pass.
According to [Smedis’ presentation][1] (slide 9) this situation causes whole FBO (to which UMG is rendered) to be copied to RAM and then reloaded.
It’s possible that you are rendering your UMG in large resolution. This will cause significant copying time. Check resolution of your UMG widgets and make them as small as possible on mobile:
