Do you have 802.11b rates disabled and have you checked if a specific client could be causing the issue? You’d need to connect 1 client at a time and then perform speed tests until you see the problem.
I guess you could also try using an OpenWrt snapshot, since that contains newer packages and more fixes.
I found this teardown and this WiFi speed comparison last night. But they only tested 5 different chips from PCs and they didn’t include phones.