If you go to system log then wireless log, you'll see the connection details between your router and devices. You'll see something like this in the bottom of the log:
Code:
Stations List
----------------------------------------
idx MAC Associated Authorized RSSI PHY PSM SGI STBC MUBF NSS BW Tx rate Rx rate Connect Time
xx:xx:xx:xx:xx:xx Yes Yes -44dBm ac No Yes No No 1 80M 433.3M 866.7M 01:08:52
xx:xx:xx:xx:xx:xx Yes Yes -49dBm ax Yes Yes No No 2 80M 907.4M 6M 12:00:01
You want to look at the PHY, NSS, and BW columns. Those will determine the maximum Tx/Rx rate.
For example, in the second row, I have AX, 2, and 80M. That means I have 2 streams and the device can use 80 Mhz bandwidth. The Tx/Rx rate columns show the established rate between the device and the router. You'll never achieve 100% of the Tx/Rx rate since you're not in ideal conditions and your router probably has to share airtime with other connected devices. 50% of the Tx/Rx rate would be a good target if you're doing a speedtest.
For AX, 2 streams, and 80 MHz, the max Tx/Rx rate you can get is 1200 Mbps.
For AC, 2 streams, and 80 MHz, the max Tx/Rx rate you can get is 866 Mbps.
Divide the rate by 2 if you have 40 MHz, and divide again by 2 if you have 1 stream.