(page) load event listeners do often something heavy. I wonder if it makes sense to delay firing load event in background tabs so that it gets fired either after 10(?) seconds or if the tab has become and stayed active for, say, 1s. Tab icon should still indicate that the page has been loaded. It is just that the event listeners would run later and also the final page loading related layout flush. (I wonder if we really have to flush layout in bg tabs before firing load event.)
We want to get rid of the flush-before-firing in general. The only issue is that last time I did that I got test failures with things actually rendering incorrectly... need to figure out why.