I've recently upgraded to Hub 4 and am now finding Facetime calls drop out within about 5 minutes of establishing the call .with "poor connection" message No problems before on Hub 3 !
Some details :-
1) problem is the same with both iphone 8 and ipad calls
2) measured wifi speed to the actual spot where I'm sat for Facetime using "Sam Knows" and 20Mbps phone to router, 98 Mbs Router to intenet - so not a real "bad connnection" at all!
Does Facetime drop when you use an ethernet cable connected device ?