those huge resolutions are STILL FRAME resolutions.
you will only get VIDEO FRAME RATES for VIDEO FRAME SIZES.
“percentages” are irrelevant. taking a numpy slice is a O(1) operation. imshow also takes negligible time.
wall clock time is acceptable here because no CPU time should be used while the driver waits for a frame from the camera. Python recommends time.perf_counter()
DO NOT expect reading a frame to take a fixed amount of time. frames are generated BY THE CAMERA AT ITS OWN RATE. the frame arrives at a specific time. if you do anything else in the meanwhile, that does not affect when it arrives.
I really oughta start writing FAQ articles somewhere… this stuff is asked like once a week.