Just to be clear, MIMO is not a new feature in 5G. LTE has MIMO. Old Wi-Fi routers from 10 years ago had MIMO.

5G is upping the number of antennas per base station. Theoretically, this allows a linear increase in capacity, but linear scaling is not always achievable in practice.

The advantage of the higher frequency bands is the larger bandwidths available there. Conventional cellular bands typically have 5, 10, or 20 MHz bandwidth, sometimes 100 MHz. 5G's millimeter-wave bands support bandwidths up to 400 MHz. Information rate scales linearly with bandwidth. That said, there are many practical challenges to operating in the millimeter wave bands that might limit real-world performance.