Nodelets have a field that specifies an update interval, in seconds. This does exactly what you'd expect, it reduces server load where possible.

Not all nodelets use this (as some show individual items that can't be cached lest users see information meant for others, while other nodelets are static and don't need to be refreshed), but Other Users does. I believe the refresh rate is 180 seconds, and there's a window of activity in which a user must have made a request or the username will not be displayed.

It's possible to hit the refresh at just the wrong time so you don't show up for a while. It was a concession the Chatterbox designers were willing to make.