Mate system monitor and "top"

When somethings looks not working well (e.g. abrowser not responding, very poor or unusable video with Jami), I am trying to check whether this is due to some overload of the computer.

I looked at the Mate system monitor and at the top command. I noticed that, in the "%CPU" column I can have a percentage largely over 100%. Also, I noticed that sometimes, "top" says abrowser is at 90% while the mate system monitor shows nothing going above 40%.

What is the accurate way to appreciate if program is not working well because of a load issue?

In general, I noticed that video communication/conference seems to be a highly challenging task on my computer (D8) and I was wondering whether it was a computer issue or a network issue. I have a fiber connection and both downloading and uploading is usually really fast, but in a videoconference using Jami with two other people, none of the two could ever see my camera. In any case, the video from them was terrible (and they said their video of each other was also terrible) and for all of us the sound was terrible until everyone switched the camera off.