I've got this too. On my Virgin Media (Cable TV) IPTV Stream box. Greenfield area with no other access points for any interference on AX86U.
Was fine on previous firmware so something has changed and no software update from Cable company either. I even switched back the firmware and it was fine. Even reset the cable company's box even though it's only a few months old.
Everything else seems OK and doesn't exhibit this issue or they cope with it better than the cable company's box. Basically, the problem is Wifi suddenly goes slow by looks of it, then recovers. No ramdomised MACS here as use MAC filter. Channel 40 5ghz. No G devices. Beacon Size 200 but tried lower but there was no change on beta firmware.
For me, Adaptive QOS (provable easily as previous version firmware speed returns immediately) and Wifi 5ghz isn't working for me whereas most releases are fine, with occasional trouble on an alpha or early beta but then is OK by final release....
I do get what Merlin says about eh can't look at every WiFi issue because so many times it is the user environment, but truly stumped here what the cause is.