What could be causing the ping spikes on the UniFi AP AC Lite + CloudKey?

Question

WiFi

We recently purchased a set of 3 UniFi AP AC Lite APs, a CloudKey controller, and a US-24 switch for a culture house. On most client machines, ping can sometimes reach 4000ms, up to the complete inaccessibility of the host. There are clients who have a stable ping of 1 ms, and such a client can be located next to someone who has a ping of 800ms. All three points are maximally spaced and their zones of action do not exactly intersect. There are no other APs within 300m of the building. All firmware on the latest hardware, connection speed is good, about 12-15 MB/s. Default settings except PMF prohibition and fast roaming. What could be the clue?