fsync is still a bit slow on BTRFS (on ZFS too, but to a smaller degree). For example, I just did a quick benchmark on Linux 5.3.0 - installing Emacs on fresh Ubuntu 18.04 chroot (dpkg calls fsync after every installed package).

ext4 - 33s, ZFS - 50s, btfrs - 74s

(test was ran on Vultr.com 2GB virtual machine, backing disk was allocated using "fallocate --length 10G" on ext4 filesystem, the results are very consistent)