This exists on multiple APs I've checked, but there's no explanation as to what's actually doing. On a Draytek AP902, this text is displayed:Welcome to VigorAP902 Speed Test.

This test allows you to find out the best place for VigorAP902. You can execute the speed test at different places of the building and select the best location for it. The performance test result is only for your reference.
When you click the Start button, it executes a Download followed by an Upload before reporting speeds for each.
In my case, most of my APs are wired back to the router, and so I'd expect them to be able to show 1Gbps Down and Up. However, the results are much lower (typically around 65Mbps and 16Mbps on the AP902). This particular AP902 isn't in a mesh, so, if it's testing via Wi-Fi, with what devices is it communicating, and how are these speeds calculated?

Saw exactly the same behaviour on 2x AP903's that were also both hard-wired...

I can tell you that the new AP805's don't have that feature so it looks like it's been dropped anyway.