I can also confirm that this firmware is causing issues.
Wireless speed went down from 500Mbps to 150Mbps on 5GHz for me as well.
I have tried resetting to factory defaults and reconfigure but the same problem persisted.
After enabling 160MHz (20/40/80/160Mhz) Channel bandwidth on both 5GHz-1 and 5GHz-2 and manually selecting Control Channel 40 (5Ghz-1) and 116 (5GHz-2) wireless speed improved a lot.
I hope it stays stable.
Wireless speed went down from 500Mbps to 150Mbps on 5GHz for me as well.
I have tried resetting to factory defaults and reconfigure but the same problem persisted.
After enabling 160MHz (20/40/80/160Mhz) Channel bandwidth on both 5GHz-1 and 5GHz-2 and manually selecting Control Channel 40 (5Ghz-1) and 116 (5GHz-2) wireless speed improved a lot.
I hope it stays stable.