Massive CPU load on idle X - PRIME configuration

[Stefan Hoffmeister]

I am looking for the right place to ask about debugging a specific configuration
* Intel modesetting driver
* nvidia closed source drivers
* PRIME configuration
* a specific 4K screen attached to DisplayPort (no issue with a 2.5K screen)

I suspect this to be an Intel modesetting problem - it allocates by
far too much memory, and it seems to continuously copy content even if
there were no changes, resulting in heaps of memory bandwidth
consumption and heaps of burning of CPU.

Alas, I am not at all familiar with the underlying software stack, so
any pointers into the right direction would be greatly appreciated.