Mxing wireless clients, AC and N, 3x3 and 2x2

[hhhd1]

Lets say i have 2 clients, one with wireless AC and 2 antennas, the other is wireless N with 3 antennas.

theoretically, max connection speed on 5GHz should be 867mbps and 450mbps.

will the router have to slow down to work with both clients at the same time ?

Will it slow down to 2 antennas speed, or to wireless N speeds ?