You've just stacked three layers of disk I/O indirection up, and you're wondering why it performs badly?
There's an old saying in computer science, attributed to David Wheeler:
Any problem in computer science can be solved with another layer of indirection.
It's not true. There is one problem you cannot solve that way: the system is too slow. (Not without adding more hardware anyway. More spindles in this case, or clustering, or load balancing, etc.)
Btrfs includes LVM functionality, but you've gone and stacked that on top of Linux's LVM2 layer and put that on top of a hardware RAID-6 system. What else have you done, hosted virtual machines on this teetering pile of abstraction, so that you can add another layer or two of filesystem indirection?
You've asked for advice, so here's mine: snap some of those layers.
If it were me, I'd configure the hardware disk subsystem for JBOD and lay Btrfs directly over it.
More broadly, you should not expect Btrfs to achieve the same performance as
ext. You aren't comparing apples to apples. Btrfs buys you a higher level of data integrity, so it will naturally run a bit slower. TANSTAAFL.