Alright folks, I managed to find a fix for the problem.
I don't know exactly why but it seems MacOs tries to do some processing with the image sent to the display that causes the high CPU usage etc.
In order to mitigate the issue, I had to select a "low resolution" mode. That means:
1. Access Display Manager
2. Select the display you want to set to "low resolution mode"
3. Set the resolution to "Scaled" while holding the option key in the keyboard. (That should list hidden resolution modes)
4. Select "Show all resolution modes"
4. Select the low resolution mode in the list, just like the image below.
Let me know if that works for you.
Cheers!