First sad thing
Every company try to advertise non existing numbers.
For example, for good USB adapter it can be AC1900 or 600Mpbs on 2.4GHz band and 1300Mbps on 5GHz band. At first, forget 2.4Ghz, in any real place it'll be so crowded that almost any good adapter or router will have similar low speeds. And you can't sum the two anyway.
1300Mbps here means that adapter is 3x3 MIMO, or has 3 separate antennas used both for transmission and receiving of signals each in separate band.
First Note! You need really almost no interference for all this to work good. Otherwise speed will start to drop really fast from limits mentioned.
Second Note! Speeds mentioned are bare theoretical speeds including all correction codes, protocols expanses, etc and have very small relation to real transfer speed you can measure.
Good case test (2x2-4x4 access points connected to 3x3 Mac Pro)
802.11ax
About not nice situation with 802.11ax draft devices.
Very long and nice paper about it https://www.arubanetworks.com/assets/wp/WP_802.11AX.pdf
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