The speed icons are generated each page load, by making a request to each server. The request comes from the client browser so generally the closer an instance is to the viewer, the higher they'll rank for that person. Everyone will get different results and depending on how much load the instance is under and general network conditions they could get somewhat different results each time.

I intentionally omitted user count because that will make people cluster together on the biggest instances, which is not healthy for the network overall. The instance-choosing websites for Lemmy, Mastodon and others all make this crucial mistake.