With lazy frame loading enabled, hidden or above-the-fold cross-origin iframes in background tabs aren't loaded until the tab comes to the foreground.

This is because, in the current implementation of lazy frame loading, cross-origin frames are all deferred until an intersection observer fires, at which point frames are individually loaded if they're hidden or near the viewport. However, intersection observers don't fire until after painting occurs, and painting doesn't happen for background tabs.

The desired behavior would be for hidden or above-the-fold cross-origin iframes in background tabs to start loading while the tab is still in the background, so that the user doesn't have to wait for them to load.