Odd.
Possible there was some network change? (The 2nd machine is on the same network? Just to point out to possibilities, maybe a new network that the PCs detect as Metered, or maybe there is some QoS/throttling on it.)
If you already went such great lengths, maybe test to see how it is on an alternative email client, like Thunderbird. Just to be able to compare.
I tested it on that dumb mail app that microsoft now pushes, and it too stood solidly empty for about 10 minutes until something finally showed up.
The 2nd machine is in a different physical location.
I called google (it's a workspace account) and they said they will escalate and get back to me, although I don't think they will because she kept assuming that a setting was wrong (imap enabled, less secure apps on, etc..) I kept insisting that everything was correct, I don't get any errors it just simply takes unbearably and highly abnormally long for things to move.
Oddly enough though, gmail on the web works fine and I tried the gmail app on the phone and it too is fine but the apple mail app it also crawls soo.... ??