Well, if you want to argue from a theoretical point of view, the fundamental constant determining the CSMA-CA slot length is the speed of light within reception

Well, if you want to argue from a theoretical point of view, the fundamental constant determining the CSMA-CA slot length is the speed of light within reception range, which is effectively constant, and the requirement to demodulate the carrier signal to perform carrier sense, which also has a fundamental time constant related to carrier frequency. Of course DIFS in 802.11 is much larger than that fundamental limit, because carrier sense needs to implemented in real low-power hardware.