On the examples page, I profiled the five InFieldLabel calls at a total of just over 28ms. Firefox 3.5, 2.2 Ghz Core Duo. Its not the field calls that take 500ms, even in Firefox 3.0.
What I profiled as taking that long was the jQuery init functions, which I assumed were required (and called by) the plugin.
Then again, I'm not as familiar with jQuery as I once was. All I know is that I don't like method chaining (impossible to set mid-line breakpoints and try/catch blocks), the extended element (if it's anything like prototype) is extremely heavy and not needed a majority of the time, and if you use massive amounts of it (see: slashdot) the user experience is deleteriously affected.
And really, 500ms initialization (even if it's jQuery and not the plugin initializing, that's a long time) for a user-experience-worsening toy like this.. I don't see the point.