This problem can be reproduced easily on a MacBook with 2 screens: the lid open + and external screen. Most people with a MacBook work this way when in the office or at home.
In my config, and also in the config of my collegue, the Jitsi window is put on the internal screen, so the external screen can be shared without any tunneling issues. My collegue has exactly the same issue, with another model MacBook.
To reproduce the problem, just share a window, not the complete screen. This avoids any confusion as of the reason of the problem. Show this shared window in the Jitsi window.
Put the Jitsi window on the internal screen. Notice CPU usage being over 300-400%. Move window to external screen. Notice CPU usage being around 100% or much less, depending on camera/sound/shared screen on other side etc.
I connected an Asus ZenScreen MB14AC USB monitor to my MacBook, and closed the lid, repeating the same exercise. Showing the Jitisi screen on this monitor is also using much less CPU.
When the MacBook is using this much CPU, it’s getting very noisy, where it even disturbs the built-in microphone with its fans. I also think it’s not very good for the machine, as temperatures crawl up to over 90 degrees Celcius. I’m not really sure what this is related to, could it be that the MacBook is using the integrated weak Intel graphics card on its built-in screen, when an external screen is connected ( and using the much stronger 2nd GPU )?