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Signal interference would be my guess.
Could be as simple as "do you have multiple APs and did you manually set them to the same channel?" if so they're probably interfering.
Likewise if you manually set the power of the AP.
As much as possible, let the AP scan and auto set the channel and power.
But... just never expect to get the full WiFi speed, if speed is important add a cable. Also be careful to get the units right when comparing things, everything in networking is bps but I see tools sometimes mix in MB/s (Windows does this in places!).
Velocio-
I've let the Unifi controller do all of the scanning & setting for me, and I've also scanned and set manually, but still not much joy.
I fully understand that wired is better, and that I'll not get the full advertised speed over wireless. But the 20 Mbps nonsense I'm getting points (or at least nods) towards something else.
NotThamesWater
It looks as though we're having similar problems.
I get 950Mbps over ethernet LAN / VLAN, across various switches.
As soon as I connect over wi-fi, though, which is supposed to be Gigabit, my speeds drop to 30Mbps.
I'm buggered if I know why.
It's not signal attenuation, as I am in arm's reach of the AP.