Thank you very much for your feedback and for sharing this with us. However I can't include it in the image as this is just a user setting (I would have to modify the default value defintions for this and I don't want to manipulate default settings of OpenWRT. My intention is to provide stable images which are absolutely close to the official ones without modified default values). One of my first steps when I install openWRT is - no matter on which router model or vendor - setting the wifi fragmentation to 2346.
For all others who experience similar problems here are the instructions how to do that:
Login via SSH and edit /etc/config/wireless:
vi /etc/config/wireless
now just add
option frag '2346'
in the
config wifi-device 'radio0'
and
config wifi-device 'radio1'
sections.
Alternativeley do it via Luci:
Go to Network --> Wireless and edit a wifi network (no matter which one). In the appearing window go to "Advanced Settings" in the "Device Configuration" section. Now enter 2346 in the "Fragmentation Threshold" field. Click on Save. Repeat the same steps for a wifi network of the other frequency band. After the configuration window is closed and you are back in the wifi network overview scroll down and click on "Save and Apply".
The reason why this setting matters:
The wifi fragmentation hat the same role like the MTU for ethernet transmissions. If you set it too small there will be a lot of fragmentations and your speed will slow down. For many devices the value is set to 2346 by default - which is the maximum value which is supported by the 802.11 standard.
By default the fragmentation is disabled in openWRT - which is normally the same like setting it to 2346. However some drivers need this value explicitly set. If other devices have smaller values you could also set it to a lower value accordingly (e.g. for wifi bridges with fixed values other than the default).
Since we're talking about wifi settings here's another setting which might help in busy environments:
option rts '500'
This enables the RTS/CTS mechanism which will cost some bandwidth but will make the wifi much more stable against collisions. Of course you can set this also in Luci (same menu as described above).