2015 MBP using external screen and built-in screen *Slows to a CRAWL* when using Google Meets and "Share My Screen"

Model is:

MacBook Pro (Retina, 15-inch, Mid 2015) with 2.8 GHz Intel Core i7, 16 GB 1600 MHz DDR3, Intel Iris Pro 1536 MB and 1TB SSD.

OS: 10.15.7.

Browser(s): Chrome (current), Firefox, Safari

This problem was the same on the last two versions of osx.

So.. What happens is:

When teaching a class of students (the district uses Google Classrooms) I launch a Google meeting (in google classroom, clearly a google meet session)... everything is fine... even when ~25 kids all log in. It does not matter if I am using the laptop's display or an external one.

Where things go sideways is when I attempt to share my screen to show the document we are working on,

...the key thing here is:

The document is displayed on the laptop's screen, not the external screen... while I have all the students in the meeting visible on the external screen (it is larger and so easier to see them all).

When i do this, the macbook pro absolutely runs into a brick wall. Instantly the kids see a monsterous lag of the screen share. There is no recovery beyond killing Chrome and logging back in with the screen share off.

This occurs with all three of the browsers I have tried: Chrome, Firefox, Safari.

It appears to be related to OSX's poor management of dual GPUs and how to mux them properly but hey, I am definitely not an OS level coder so.. that guess is not something to hang your hat on.

This computer cost me **$3600** new! It was a HUGE investment. All the bells and whistles. And yet it is humbled by the extremely CRAPPY GPU/display management.. which has not improved...even though I have kept OSX (now MacOS) up to date.