Thanks Laurenz, that confirms what I was assuming. Archiving is via pgbackrest to a backup server, over SSH. Approx 750ms to archive each segment is crazy -- I'll check compression parameters too.
Switch to archive-async = on. When doing that, the typical time drops to 10ms or less. Also use a compress-type of lz4 or zst, which perform way better than the default gz. If you are encrypting, that's a bottleneck you just have to deal with, no shortcuts there. :)
tl;dr try other things before messing with the WAL size. The current size can work very well even on very large and very, very busy systems.
