Getting full speed using hostapd as a wifi AP.

I'm running Debian 9.5 with hostapd as a wifi access point. Everything works fine except all I'm able to get is about 20mbit throughput over wifi. The wifi card in the server is a TP-Link TL-WDN4800 N900. It's capable of doing 450mbit.

I'm not sure if this is a hostapd issue or the driver for the card issue. Below is my hostapd file. Any help in getting this card to full speed or at least to average 802.11n speed would be appreciated.

driver=nl80211
hw_mode=g
channel=6
beacon_int=100
wmm_enabled=1
ieee80211n=1
ht_capab=[HT40-][HT40+][SHORT-GI-40][TX-STBC][RX-STBC1][DSSS_CCK-40]

I left the ssid and key out for security reasons.

You're getting ~2.5 megaBYTE transfers? Because 20 megaBYTE transfer seems normal to me. The bottle neck is the usb/pci circuitry, at least last time I looked into it.