I was wondering if someone has such problem with this router (or any other with MT7620N chipset probably).
Though very stable, but slow wireless. Currently I`m on latest snapshot with MT76x0 patches.
My ISP is 100/100 Mbps and I get this speed on LAN connection. On wireless its only ~25/20 Mbps. Using HT20 with psk2+ccmp.
Tried different channels, devices, tx power, distance, you name it
From that information and probably a medium to ok signal, you're best is about 60% of the MAX PERFECT speed which is 54Mbps, it comes down to about 34.56Mbps. That's very close to the ballpark figure of max throughput. You could probably look for gso, gro, or tso in ethtool and mess with interrupt coalesce plus tuning the channels. Otherwise. that's the best it can be for that protocol. Using WEP would lessen the overhead as well, but not recommended due to security reasons.
What device are you currently using to access this AP? That device's client protocol will define the bandwidth it can utilize. An example is a router that supports AC protocols (1300Mbps) server but you have an older laptop such as a 2013 Macbook Pro client, which it's WiFi card only supports WiFi N (Max 600Mbps). You get the lower speed and about 60% of the max (360Mbps) transfer speed. Many factors affect the outcome but that's the general gist of it.