[vlan0]

How do you think speeds effect airtime utilization/optimization? And how does this change with lower PHY rates?

[immibis]

Not linearly. The data may take twice as long to transmit at half the rate, but there's overhead which doesn't change. If you can use half the channel width and transmit at half the rate, you halve the Hz-seconds used for preambles and stuff, while keeping the Hz-seconds used for data the same.