Virtual disk image fragmentation
Speed, while crucial, is not the only thing to care. Fragmentation at the host-system level is also a very important factor, as an highly fragmented HD image file will become slower and slower over the time.
So, what filesystem is the better in contain fragmentation? Let analyze HD image file state after IOMeter tests:
Yeah, you read it correctly: while XFS is the absolute champion in reducing fragmentation and EXT3 / EXT4 remain at acceptable level (albeit much worse then the leader), BTRFS is shockingly bad in this regard. The virtual disk image file was fragmented into almost 300K fragments!
I know that BTRFS had a fame to be very fragmentation prone,  , but I hoped that during latest kernel development this problem had been corrected or reduced, but this don't seems to be the case.