BTRFS Rebuild time doesn't make logical sense!

BTRFS Cache pool containing 6 drives.  Removed a old slow 500gb drive (ensuring lots of spare space...over 2 TB free in the pool, over the size of the removed 500GB drive) leaving 5. Rebuild started automatically (as expected) has been running faithfully now....for a F-ING MONTH!!!  I know it takes about 4 hours tops to move 500gb of data (on really slow hardware)...why is this taking months to rebalance a small pool??? 

This has happened with other pools on other machines as well, and one time taking 3 months to finish...all the while, making read/write time almost unusable for normal operation...if i cancel the rebuild, i risk losing the pool all together (experience taught me that silly lesson).  

This would have been 1000000000x faster if i moved all the data to another pool, rebuilt this pool and started from scratch every time i want to add, replace, or remove a drive!  Is that normal for BTRFS management or is something wrong (it better be, cuz this time spent isn't ok).

I don't know if ya'll would want the full diag, or something more specific to this pool...so i will await a gurus advice!