Edimax RA21s poor performance

Hi all,

yesterday I replaced the stock firmware of my access point with OpenWrt because I was unhappy with the featureset and the limitations of Edimax' software. Installation was quite easy and I had my final setup after a couple of minutes. Great!
Unfortunately, the performance is poor. Very poor compared to the stock firmware.
But let me first describe my setup:

specs
  • used as AP only
  • no WAN, NAT, packet filtering etc. features
  • no VLAN features (yet)
  • the AP is connected via 1GE to a Draytek switch
  • the router/gateway, a Linux server, is connected to that switch via 10GE
  • 2.4GHz and 5GHz with the same ssid
  • 2.4GHz mode N on channel 8 (which is not occupied by other wifi devices), width 40MHz, power 100mW
  • 5GHz mode AC on channel 124 (free), width 80MHz, power 199mW
  • several other networks are around but none should conflict with mine (I use the same settings as before with the stock firmware and the neighbour's networks didn't change)

With Edimax' firmware, I got something around 500Mbps to 800Mbps tested via iperf3 from my wifi client (laptop) to the gateway. With OpenWrt, I get 1/6 of that, at best.

So I tried some optimizations, like

  • narrowing the bandwidth of the 5GHz channel from 80MHz to 40MHz
  • enabled software and hardware offloading (even though no nat is done here)
  • disabled unneeded services: dnsmasq, firewall, odhcpd
  • disabled network steering
  • raised net.core.{w,r}mem_max to 16777216
  • raised net.ipv4.tcp_{w,r}mem to 10240 131072 16777216
  • raised net.ipv4.udp_mem to 10240 131072 16777216

I installed iperf on the AP and did some measurements (only the best results shown).
These tests are against 5GHz, the 2.4GHz network is even worse.

iperf speedtests network steering ON
server client mode client -> server (Mbps) server -> client (Mbps)
gateway ap unidirectional 750 -
gateway ap bidirectional 575 375
ap gateway unidirectional 875 -
ap gateway bidirectional 600 360
wifi client ap unidirectional 140 -
wifi client ap bidirectional 120 15
ap wifi client unidirectional 270 -
ap wifi client bidirectional 40 6
gateway wifi client unidirectional 250 -
gateway wifi client bidirectional 105 35
wifi client gateway unidirectional 115 -
wifi client gateway bidirectional 145 85

Note: the wifi client (laptop) is like 1.5m from the access point.

iperf speedtests network steering OFF
server client mode client -> server (Mbps) server -> client (Mbps)
gateway ap unidirectional 930 -
gateway ap bidirectional 800 50
ap gateway unidirectional 850 -
ap gateway bidirectional 650 360
wifi client ap unidirectional 160 -
wifi client ap bidirectional 145 35
ap wifi client unidirectional 260 -
ap wifi client bidirectional 155 22

(edit: tcpdump looks fine. No errors, drops, retransmissions, dup acks, etc.)

Could anyone point me please to some settings to get close to the original firmware's performance?