Chrome DevTools: Slow frames, but no events causing them

If you click on the time-span above the main thread, and then click on the Bottom-Up tab at the bottom, you can see all the individual activities going on that add up to that total time. I suspect there must be some other activities you can't currently see, but if you scroll vertically, you should see them - probably Rasterize Paint on another worker.

UpdateI realised that the times in the Bottom-Up view do not add up to the total time shown for the frame. I did a little more playing and it looks as though the remaining time is 'Idle' time.

If you look at the screenshot below, I have adjusted the Timeline view to contain approximately one frame that has very minimal activity. It's a little bit more than one frame because you can see the dotted lines on either side are a little further in.

If you then look at the bottom Summary view, you can see the majority of time is listed as 'Idle'. If you pretend I more accurately filtered the Timeline view to be exactly one frame (by taking off a tiny bit of time from the summary values), you could probably fairly confidently come to the conclusion that the total frame time (shown just above the Main Thread bar) is the summation of the values shown in Summary, including the 'Idle' time.