Mine is 30 gigabytes or so. Using mbsync, it takes up to half an hour for the first sync, then 10 seconds or less to sync after that.

I do that once every minute using Cron, and it has been serving me quite well.

I don't usually open up my archive folder, when I do it takes quite a lot of time to open on neomutt at least. Notmuch works really well after indexing though.

Mine is 7GB and the tool has no issues, don't see a reason it wouldn't scale.

On my SSD ripgrep takes about 5 seconds in cold start to search over all of it, then less than a second after that (presumably because drive caches have populated). It may be enough if you only search mail infrequently, otherwise perhaps Recoll [0] indexer might help, I believe it can handle .mbox files (so each message would be indexed as a separate document)

[0] https://www.lesbonscomptes.com/recoll/pages/features.html

I wrote a cross platform Desktop app that can analyze these email backups to provide a visual clustering of the contents of your mails.

https://github.com/terhechte/postsack

It parses my 13gb Gmail backup in under a minute.

notmuch can index such mail archives and then search them in seconds.