I loaded a playlist, but Viridian takes forever to show it.

This is most likely due to the fact that the information for every artist/album/song in the playlist is not cached locally in the Viridian database. (read the faq on caching to understand more about how Viridian caches information).

Everytime the playlist window needs to update (like you select a new song, or the song finishes and moves on to the next), Viridian queries the Ampache server for every song in the playlist that does not have information cached locally. To decrease the loading time for a playlist you can find every artist/album in the playlist and click them to load their information in Viridian, or simply do File -> Pre-cache (read the faq on pre-caching for more info).

This should only affect loading a playlist for the first time.