This question is really for TP-Link support.
On the EAP660HD, even though this device is 4x4 MIMO (on both 2.4 and 5GHz) the reality is that despite MU-MIMO and OFDMA, this device cannot simultaneously pass data from two separate 2x2 MIMO clients (STA's) at full speed.
For example, given my EAP660HD at home (Wi-Fi 6, 4x4. 80MHz, 5GHz, OFDMA enabled, 1 Gigabit/s DL/UL FTTH) and my mobile phone (iPhone 13 Pro Max, 2x2, also Wi-Fi 6) when I run Ookla Speed Test from one STA I get approximately ~550-700 Mbit/sec DL and UL from optimal Speed Test servers. For the sake of round numbers, let's call this ~600 Mbit/sec.
If I then perform the same test using TWO STA's (e.g. two separate iPhone 13PM's) each mobile client only seems ~250-300 Mbit/sec DLand UL, That is to say, each client gets roughly half of the usable (aka Application layer) airlink bandwidth.
BUT, what should be happing, is that because the EAP660HD is a 4x4 MIMO device, it SHOULD be able to support TWO 2x2 STA's at full speed (aka 2x600 Mbit/sec) up to the limits of the AP's CPU and ethernet uplink bandwidth (which in my case would be 1 Gbit/sec)
This is a well known issue/limitation (on Reddit and the interwebs) -- Google search the following review from HWP Media.
"We turned to TP-Link for clarification, and we were informed that Yes, today there are difficulties with the distribution of clients between spatial streams in the 5 GHz band, and this error will be corrected literally in the next firmware version,"
This review pointed out this limitation in December of 2020. But still this limitation is not fixed. Will it ever be fixed?