Best Practice: Add new device to RAID1 pool

[Cloud Admin <admin@cloud.haefemeier.eu>]

Hi,
I have a multi-device pool (three discs) as RAID1. Now I want to add a
new disc to increase the pool. I followed the description on https://bt
rfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices and
used 'btrfs add <device> <btrfs path>'. After that I called a balance
for rebalancing the RAID1 using 'btrfs balance start <btrfs path>'. 
Is that anything or should I need to call a resize (for example) or
anything else? Or do I need to specify filter/profile parameters for
balancing?
I am a little bit confused because the balance command is running since
12 hours and only 3GB of data are touched. This would mean the whole
balance process (new disc has 8TB) would run a long, long time... and
is using one cpu by 100%.
Thanks for your help and time.
Bye
	Frank