I did a Cycles X ‘scalability’ test that I call - Expectations vs. Reality. I took my scene and rendered it at different resolution and then I compared results to what I was expecting. I used 1080p as a baseline and based my expectations on the number of pixels. I decided to post the results because they might be useful. To make things easier to read, I rounded some numbers. You can scroll to the conclusion to see the expectation vs. reality result:
Resolutions:
4320p: 7680 x 4320 (33 177 600 px)
2160p: 3840 x 2160 (8 294 400 px)
1440p: 2560 x 1440 (3 686 400 px)
1080p: 1920 x 1080 (2 073 600 px)
720p : 1280 x 720 (921 600 px)
Difference against the baseline (Expectation):
4320p: +1500%
2160p: +300%
1440p: +78%
1080p: 100%
720p : -56%
Results CUDA (Time in seconds):
4320p Expected: 326.56 (+1500%)
4320p Reality : 398.52 (+1853%)
2160p Expected: 81.64 (+300%)
2160p Reality : 93.58 (+359%)
1440p Expected: 36.28 (+78%)
1440p Reality : 35.43 (+74%)
1080p Expected: 20.41 (100%)
1080p Reality : 20.41 (100%)
720p Expected: 9.07 (-56%)
720p Reality : 8.73 (-57%)
Results OptiX (Time in seconds):
4320p Expected: 319.20 (+1500%)
4320p Reality : 399.32 (+1902%)
2160p Expected: 79.80 (+300%)
2160p Reality : 91.60 (+359%)
1440p Expected: 35.46 (+78%)
1440p Reality : 35.17 (+76%)
1080p Expected: 19.95 (100%)
1080p Reality : 19.95 (100%)
720p Expected: 8.86 (-56%)
720p Reality : 8.58 (-57%)
Conclusion:
4320p: 24% slower than expected
2160p: 14% slower than expected
1440p: As expected
1080p: As expected
720p : As expected