Ok, then I guess by disabling WMM you probably throttled your wifi and hence reduced the chance that other people on wifi interfere 
Here's a link to an example script: Creating DSCP markings with iptables? - #4 by dlakelan
In addition to setting up DSCP you can set up a layer-cake instance on your wifi interfaces and set the bandwidth there say 10% lower than you use on your WAN, this will reduce the chance that people on your WiFi will hog the whole WAN, without disabling the high speed features that require WMM
Remember that on your WiFi, the egress should be set at say 10% lower than your WAN download speed, and the ingress at 10% lower than your WAN upload speed. It's reversed from what it is on the WAN.