How to improve the wifi performance of openwrt

Question

If my computer download through my tplink router, the speed is 10Mbps, if through openwrt router, it's 5Mbps.

Here is wifi information:

root@localhost:/etc/config# iwconfig wlan0
wlan0 IEEE 802.11bg ESSID:"AndroidAP"
Mode:Master Frequency:2.427 GHz Access Point: 00:8E:0E:02:99:4E
Bit Rate:54 Mb/s
RTS thr:off Fragment thr:off

Encryption key:off
Power Management:off
Link Quality=0/100  Signal level=-95 dBm  Noise level=0 dBm
Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
Tx excessive retries:0  Invalid misc:0   Missed beacon:0

If no traffic, the system information is as follow:

Mem: 689272K used, 299240K free, 0K shrd, 10228K buff, 308060K cached

CPU: 4% usr 2% sys 0% nic 92% idle 0% io 0% irq 0% sirq

If traffic is 5Mbps, it's

Mem: 692320K used, 296192K free, 0K shrd, 10268K buff, 308060K cached

CPU: 3% usr 2% sys 0% nic 89% idle 0% io 0% irq 3% sirq

The CPU is nearly idle.

I haven't install qos package, there is no qos config in /etc/config

I don't know why the WiFi performance of openwrt is so bad.

Please help me to improve WiFi performance.