slow download link with hostapd and ath9k_htc driver

Brief Intro:

Raspberry Pi 3 acting as a WiFi AP hosting 2.4Ghz and 5Ghz Wifi AP's. The AP of concern is the 2.4Ghz network. The NIC is a TP-Link WN722N adapter using driver ath9k_htc.

Tests:

A speedtest done from the Raspberry Pi 3 results in:

• Download: 21.69 Mbit/s

• Upload: 18.32 Mbit/s

which is expected and correct.

Clients connecting to the 2.4Ghz hosted network doing a speedtest results in:

• Download: 1-11 Mbit/s (AVG: 2.5 Mbit/s)

• Upload: 14-18 Mbit/s

note: Changing router channels does not have any effect on the download speed of hosted wifi network.

A shared ethernet connection, hosted by the Raspberry Pi, to which clients connect do not experience this issue. These achieve expected download speeds.

Question:

Since all NIC's report correct download and upload speed except the hosted wifi network (2.4Ghz adapter) using the ath9k_htc driver has slow download speed, is this an issue with the ath9k_htc driver?

Some HW Info:

note: The hosted 5Ghz network does not have this download issue, speeds are correct and as expected

NIC Info

and hostapd config