David Chartier, writing about his sea change experience going from watchOS 2 to the watchOS 3 beta:

A big help are the new tools in the iPhone app which make customization of Watch faces and complications much faster. But the real star, for me, is the huge performance and speed increases for third-party apps, including the new quick-switching Dock that takes the place of the previous friends wheel. As far as I know, developers cannot yet publicly release updates for watchOS 3 (whereas a number of iPhone and iPad apps have already updated in the App Store), but even current apps start up and simply run better. In a word, they’re usable now.

This is definitely my experience as well. David make’s an interesting point here: The big performance change is purely based on existing app code, not on code tuned specifically for watchOS 3.