Are you using qgroups? You can verify it by "btrfs qgroup show -pcre <mount>" to verify. Qgroup can hugely impact performance for balance, if using kernel older than v5.2. You can either disable qgroup, losing the ability to trace how many bytes are exclusively used by each subvolume, or upgrade to v5.2 kernel to solve it.
Not using qgroups. root@wawel:~# btrfs qgroup show -pcre / ERROR: can't list qgroups: quotas not enabled Current data transfer speed is 2Mb/sek.
