Anyway, it is apparent that issue is not merely a linear increase in directory operation time, but a for-real bug that looks like some kind of live-lock. The hash can and should be fixed, because although the attack in question turned out to achieve something other than its original intent, there is no question that trivially being able to force hash collisions is a vulnerability.
The most important result here is not the inappropriateness of CRC as a hash, but that Btrfs is a complex beast, still in development, a critical bug was just exposed, and likely a few more remain. Given wide enough testing and continued developer commitment it will become stable, but today it is not. From where I sit, it looks like Ext4 will be wearing its standard Linux filesystem crown for some time yet.