I had a slow startup too. I installed Kubuntu 11.10 choosing btrfs as file system, and noticed was booting far slower than ext4. I asked in IRC channel and Chris Mason (wow!) troubleshooted it (how incredible is FOSS world?).
[20:33] <cmason> ok, start with the fstab
[20:33] <cmason> UUID=d5acc217-d524-4a2d-a937-bad945a047b2 / btrfs defaults,noatime,nodiratime,subvol=@ 0 1
[20:33] <cmason> last number should be a zero, should just change fsck.btrfs to a noop
fsck is slow, because it's not intended as a boot-time check
In fact after this change my atom single core boot time dropped from 111 sec to 80 sec