At last I seem to have identified a replicable version of the transitions problem that we saw earlier in the term. It seems to occur specifically on chrome for Android. I've replicated it on my own Galaxy S7 after a student showed it on her J5. If you try completing a live quiz, the transitions between questions are incredibly slow, and the screen is blank for much of the time. It can take 30 seconds for a question to appear, and seems to get worse as the quiz progresses (though that may not be a consistent effect).