Mac book pro 16 slows down completely when connected to an external monitor
After spending weeks of trying out various things: different HDMI cables, different USB-C to HDMI dongles, cleaning the fans and even leaving the air conditioner on to cool the room, the issue turned out to be:
Plugging the power cable on the left-hand side of the Macbook.
Solution:
Plugging the power cable to the right-hand side of the MacBook and the monitors to the left-hand side, solved the issue for me immediately.
Came across this Forbes article:
https://www.forbes.com/sites/barrycollins/2020/04/24/why-you-shouldnt-charge-your-macbook-pro-from-the-left-hand-side/?sh=6209dcda78ff
Which I can now say, works. So if you are experiencing something similar and if your power cable is on the left hand side, moving it to the right hand side may fix it for you.
The other thing I would try (which I did) was to clean out your fans if this still does not work for you.