Upgraded Firmware from Archer C4000(US)_V2_190827 (2019) to 220114 (2022) and it is noticably affecting download speeds over wireless. Wired connections are not affected, and speedtest from router is not affected, but wireless clients have dog-slow download speeds. Did all the usual troubleshooting to no avail. Re-flashed the router with the 2019 firmware and now the wireless clients are working at high speed. It makes me think there's a bug that was introduced in the newest firmware.
Archer C4000 V2 Latest Firmware -> Slow Wireless Internet
Model: Archer C4000
Hardware Version: V2
Firmware Version: Archer C4000_V2_220114