I actually hit this for the first time in all my years of using btrfs a few weeks ago.

Every month or so I spin up external drives to write out monthly backups of my NAS and personal computers. Forgot I'd taken on the home video collection and overfilled the array. Deleting some old subvolumes and having the space reclaimed took hours (on a 99.9% full 5 TB RAID-1 volume). Part of that is that I use compress=zstd:10 on the external drives, but I wasn't expecting it. Writes speeds tanked to tens of KB/s. Since these are those Seagate drives from Costco, probably also SMR drives...