First, I must preface by saying the tag cloud (fatcloud) looks absolutely beautiful. But the plugin is incredibly slow. It slows down my page significantly on every load, even with cache enabled. With cache enabled, it will slow down even more every single time I add a post (thus adding tags). Without cache enabled, every page load is slowed by several seconds. In my case, it was slowing me down by as much as 27 seconds on some loads.
I like it, but it would need to be more speedy for me to put it into production.