Late to the party - and sorry to reopen an older thread - but I have the same problems as the OP does. 8 Amcrest cams, 7 of them wifi, with various of the wifi ones experiencing dropouts and very poor throughput at times. My solution was the same as mentioned - wire them up. Even the one 15 ft away from me just outside, playing dropsie. I have 3 wifi cams left to do, and 2 of them right now are doing 30-45kB/s while the wired ones go anywhere from 400-1000kB/s.
Now, this 50x25 house is on a native reservation and I got nobody around using wifi but me. The nearest house is 500ft away. None of my inlaw neighbors use wifi, so interference should be at a minimum right? Well, maybe. Keep FCC Rule 15 in mind.
The cams are pretty chatty all by themselves, of course. But even out here there's 2 TVs doing wifi, a Mac running Plex, 2 iPhones & iPads, Kindle, Echos, a laptop at times, cell repeater, PS4 etc. The wifi environment is more crowded than you might at first believe. Changing routers didn't change things much, in fact made some things worse (and some better). Despite my attempt to blame it all on a native burial ground, the problem was obvious - so off I wire. At least there's no HOA here to complain about my wiring runs ...
I don't know if the issue we were both seeing with Amcrest cams is unique to them, or exists in some combo with BI, or what. But I do know that when I connect via wifi to the Amcrests' web page most tend to prompt for an addon to be installed to see the video. When I hardwired them, the prompt went away and I could see live video. Make of that what you will.