I've narrowed down this issue to the awdl0 virtual network interface, which is how macOS represents the ad-hoc WiFi connections it establishes for Continuity features like AirDrop. I believe this stops functioning when Bluetooth is turned off, because the features use both WiFi and Bluetooth in tandem. Similar to what happens when the OS scans for WiFi networks, when this interface is used for AWDL, it has to "steal" the WiFi antenna briefly. Not enough to disrupt the connection, but enough for us to notice the ping spikes and varying limits on bandwidth it causes.

On the systems I've tested, disabling the awdl0 interface stops the OS from carrying out AWDL-related tasks, leaving the WiFi chip undisturbed. You can test this yourself by running "sudo ifconfig awdl0 down". The ping and WiFi throughput should stabilize. 

There's lots of documentation of folks running into this since Yosemite: https://medium.com/@mariociabarra/w...t-yosemite-wifi-and-awdl-airdrop-41a93eb22e48

You'll find most of the reports on subreddits and forums for real-time applications, where users would be most likely to notice the kinds of subtle issues that this issue causes. Google Stadia actually recommends disabling the awdl0 interface to resolve connectivity issues on macOS: https://support.google.com/stadia/answer/9595943?hl=en#zippy=,mac-and-ios-troubleshooting

I documented this in a bit more detail in a Reddit thread, including system logs showing the AWDL activity correlated with the ping spikes: