[cmurf]

I haven't heard this and I've been on the btrfs list for years. The biggest issue you're going to have at 80+% usage, that applies to all file systems getting slow, is transfer rate drops due to inner tracks having fewer sectors; and depending on the workload, seeks are increasing simply because there's more stuff on the disk to go looking for.

As for fragmentation there are two kinds: fragmentation of files into more than one extent, fixed with 'btrfs filesystem defrag' and also autodefrag. And the other is fragmentation of free space, as a result of deleting files and that's fixed with 'btrfs balance' which consolidates extents and writes them into new chunks then frees up large regions of contiguous space on the drives. This is best used with filters.