What? You have to schedule it, they literally wait 3 days before they start it, and then it can take most of a day to get it ready for download. It is not fast.
FWIW, for several years I've tried backuping my gmail account with imap (including some stuff made specifically for gmail): It never succeeded. The best syncer were running for one month, and after one month it hit some mails that it simply couldn't retrieve? Like I guess it was in too cold storage and timeout-ed? I don't know.
So I can understand why using Google's proprietary API might work better (or not, I don't know)
Anyway, as a sibling says, nowadays Google Takeout includes mbox and work properly (and is pretty fast, like half a day), but doesn't allow continuous update.
And I migrated to another mail provider (infomaniak), and I've thanked myself for using my own mail domain name years earlier.
I had the same problem when I switched off Google. I didn't have a ton of data, and I just wanted content for past search purposes, so I didn't dig into how the data would be transformed but I can at least offer my scuffed solution.
I installed a third-party client (Thunderbird, but I imagine any would work) on a local box, signed in with both emails, and just copied the mail over from one to the other. Low-tech, but it worked quite well. I may have forced some local cache/download for the original email, but I can't recall. I'll check later if it preserves headers and the like. I assume it would, but it wasn't that important to me.
I actually thought about writing at some point about the process of getting off gmail and all the funny things I ran across.