The QGC version needs to be matched to the firmware version on the autopilot. This is why you get warnings about parameters missing sometimes, it means that the firmware on the autopilot has some changes that QGC is not aware of.
Using the latest software on both the laptop and the ROV will resolve this issue.
The video lag issue is a problem on a small fraction of computers running windows, but there are steps that can be taken to resolve the issue. I will post some troubleshooting tips to your topic directly regarding the lag.