Here's how long it takes me to perform a balance operation currently. In this attempt I did convert to RAID1, however as you can see, the operation takes some time (3.8 days):

time btrfs balance start -dconvert=raid1 -mconvert=raid1 /srv
WARNING:

        RAID5/6 support has known problems and is strongly discouraged
        to be used besides testing or evaluation. It is recommended that
        you use one of the other RAID profiles.
        The operation will continue in 10 seconds.
        Use Ctrl-C to stop.
10 9 8 7 6 5 4 3 2 1
Done, had to relocate 849 out of 849 chunks

real    5590m16.646s
user    0m0.000s
sys     648m28.856s