Yeah, it probably isn't your problem, but your 2-3% CPU usage suggested to me you weren't analyzing every frame (if you care). My i7-6700 went from 2-3% CPU usage to 20-50% at 15fps for four dual stream cameras. Just trying to save you some head banging on the wall if you were to ever notice it. It drove me nuts seeing live views updating every 4s and motion triggers completely missing people walking by. But fingers crossed it also helps your problem.
I have also updated the firmware of the cameras. Mine were updated to a release on 10/31.
When setting up the cameras, BI didn't detect my make/model of cameras. I manually selected ReoLink/*RLC-410/411/420/422/423 Baseline RTMP and I think manually changed the suggested RTMP URL's between 'main' and 'sub' as needed. This seems different than what ReoLink suggests on their site. I'd check that out and try different suggestions out there on the interwebs as well.