With the Btrfs pull request for the Linux 3.12 kernel merge window are a large number of fixes, performance improvements, and clean-ups.
As noted by Chris Mason in the pull request, there's also subvolume UUID indexing to speed up send/receive with many sub-volumes/snapshots. This indexing works around a performance issue that ended up getting baked into the Btrfs disk format.
Another committed feature for Btrfs in Linux 3.12 is de-duplication (dedup) support while the file-system is mounted and active but not being done inline during file I/O operations.
Btrfs benchmarks on the Linux 3.12 kernel are forthcoming on Phoronix, plus other interesting Linux kernel benchmarks.