https://askubuntu.com/questions/836105/ubuntu-16-04-takes-long-time-to-boot-using-btrfs-and-persistent-logs suggests disabling COW on the systemd journal directory may help.
Avoiding / as btrfs, and using it just for e.g. /var/lib/lxd, may be an option for some people.
https:/ /askubuntu. com/questions/ 836105/ ubuntu- 16-04-takes- long-time- to-boot- using-btrfs- and-persistent- logs suggests disabling COW on the systemd journal directory may help.
Avoiding / as btrfs, and using it just for e.g. /var/lib/lxd, may be an option for some people.