Team chat, the Design editor, the Document editor, recording sessions and the progress bars on background jobs all keep asking the server whether anything has changed. Until now they asked at the same rate forever, whether you were working in them or had left the tab open since yesterday.
They now ask just as often while you are using them, slow down after a minute or so without any interaction, and stop entirely while the tab is in the background or the window is covered by another window - which the browser reports the same way it reports a background tab. Coming back to the page picks straight up again: one catch-up request brings back everything that happened while it was quiet, so nothing is missed, only delayed.
Things that genuinely cannot wait are exempt. A recording session in progress and the signalling behind a live call keep their full rate even with the tab hidden, because that page is doing the recording and is still in the call. Chat keeps a slow background heartbeat so a new message still reaches you before you switch back.
On a laptop this is the difference between a parked tab making a request every couple of seconds all day and making none at all.