Millimeter wave spectrum matters, as a quick comparison of mobile network speeds shows. Verizon, basing its 5G rollout initially on millimeter wave spectrum, handily beats the other U.S. networks using 2.5 GHz, 850 MHz or 600 MHz frequencies. It is just physics: low frequency spectrum has better coverage, but high frequency provides the greatest capacity.
That noted, for most customers, experience will be a combination of 4G and 5G network access. That evens out typical speeds. New T-Mobile will have an opportunity to challenge Verizon on that score.
Since most mobile operators globally are launching 5G using mid-band spectrum, typical speeds will not reach the levels of millimeter wave deployments.