What is meant by the term 'time selectivity' of radio channels? And how is it much faster for mm waves compared to radio waves?
At high Carrier bands and high data rates with shared RF Synthesizers, it is faster for the RF Synthesizer settling time to use the same channel for TDD rather switching synthesizer freq from demod to mod with FDD. 

The time slots do not have to be equal for each side and asymmetry of bandwidth for each side is allowed.  Also spectrum for FDM is double so as spectrum costs rise this doubles the cost, but range limitations will require more cell towers. Like Ethernet return times and distance are limited. In Telecom we call this " Line build out " or return delay times which must be critically controlled for mobile to be received with the same delay while moving with short time slots so it is not a power limitation, rather a time slot sync doppler limitation.