iPhone/iPad Wi-Fi becomes very slow when bluetooth is on

I'm experiencing a very slow Wi-Fi connection on all my devices whenever bluetooth is on. All my devices run on the latest software (iPhone Xs Max, iPhone 12, and iPad Pro 11" 2018 - iOS/iPadOS 14.7.1).

I've tested this numerous times by trying to compare the internet speed by turning the bluetooth on and off, and the results are always the same: When the bluetooth is off, I get approx. 70 Mbps, this is very close to my subscribed internet speed which is up to 75Mbps. But every time I turn the bluetooth on, the speed drastically drops to around 5-10 Mbps only.

I've also tested the speed on my Mac and iPad Pro using ethernet cable, and the speed was normal, 75Mbps.

Somehow the bluetooth is interfering with the Wi-Fi. This never happened before. I did try and test it with another iPhone Xs Max running on iOS 14.3 and the results were the same.

The router I'm using is Cisco DPC-3828s.

Does anyone has a fix on this? I hope this problem will be fixed in the next iOS update.