The reason why 40 fps feels better is because even though its only 10 fps higher, the frame latency is half(25 ms) compared to 30(33.3 ms) and 60(16.6ms) on the two ends. So you work 33% harder for half the latency.
To put in perspective, the drop in latency (8.3 ms), is the same for example going from , 40 to 60, or 60(16.6) > 120(8.3 ms)
The reason why 40 fps feels better is because even though its only 10 fps higher, the frame latency is half(25 ms) compared to 30(33.3 ms) and 60(16.6ms) on the two ends. So you work 33% harder for half the latency.
To put in perspective, the drop in latency (8.3 ms), is the same for example going from , 40 to 60, or 60(16.6) > 120(8.3 ms)
I love how technical the answer is