Archer C2v3 - bad video performance


I am having a peculiar issue with my archer C2v3 (19.07.5 and .6). General performance is very good (compared to my previous hardware), speedtest gives 170-200mbps of my 250mbps connection.
However when watching videos on reddit on iPad Air 2 (iOS 14.4) or Pixel 2 XL (Android 11), the videos take several seconds to start playing and the quality is very low (doesn't matter if it's via app or browser). Watching videos on youtube is no problems, browsing web is neither. However when connected to another AP (archer C2v1, stock fw), the issue goes away. Issue is also not present on iPad Air 1 (iOS 12.5.1, only .11n dualband)
Do you have any ideas how to debug this, find out the cause?