Comment # 105 on bug 1017461 from Flex Liu

Recently, I encounter a btrfs snapshots remove/cleanup issue with a huge
snapshots list. if you have over hundreds snapshots, remove snapshot would let
the system freezing for a while by btrfs-transaction 100% cpu cost. it is quota
unrelated.

According the upstream explication:
http://www.spinics.net/lists/linux-btrfs/msg57956.html. it reasonable that as a
result of this, the work to create a snapshot only depends on the complexity of
the directory structure within the subvolume, while the work to delete it
depends on both that and how much the snapshot has changed from the parent
subvolume.