We had an interesting issue yesterday whilst undertaking some trials with our BlueROV2 Heavy and ping360. We experienced severe video latency (>2s) and also some control latency as well.
To assist others in the future, I have documented what we did and how we resolved this.
Initially we thought it might be because of QGC 4.0.X and some of the video issues mentioned elsewhere on the forum, so we tried a variety of builds including older 3.8.x builds - but the problem persisted. We then looked at network speeds and noticed that the upload was normal (~80Mbps) but the download was a terrible 25 to 30Mbps. This seemed to be the issue (we also looked at using the dedicated NVidia GPU for QGC etc, this didn’t seem to be a video encoding / processing issue but a network bandwidth issue.)
We then realized that this was the first time we have run the system on laptop without mains power. We plugged laptop into an inverter and the issue resolved. So we thought this must be to do with USB power to the FXTI, we rigged up separate power to the FXTI and tried again, but when laptop on battery only, the issue returned.
We found the issue in the advanced power management settings in windows and by turning off PCI Link State Power Management when on battery mode and a quick reboot, all was resolved. We have tested this repeatedly now back at the lab and it is repeatable.
Hopefully this might help others if they encounter this (Dell G3 laptop for reference, with Win 10).