When a page is backgrounded, it can become throttled, it will never paint, etc. In general, browsers apply heuristics to mitigate the impact of backgrounded tabs on performance on the visible tab. However, this affects aggregation of Performance APIs: values computed during a background status are not really comparable to values computed during a visible status because the browser can behave differently in these cases.

Loading is the most affected because (1) most performance metrics involve loading in one way or another and (2) it's common to be backgrounded while loading. I think the most common case of this would be opening a new Chrome with multiple tabs: all but one will load in the background.

A prominent example of this is PaintTiming. The paint timing timestamps are only computed once the process actually paints, which means that the timestamps can be inflated arbitrarily while the tab is in the background.

Filing this bug just to make sure we don't forget about this problem.