This is because in Java land startup speed isn't viewed as something important.<p>Groovy also has the poorest runtime IMHO. JRuby or Clojure are in a better shape.
I love app engine and if it had SSL I'd use it for all projects I do ... BUT: It feels odd to run the speed analysis tool in google webmaster tools and discover that my very simple site is slower than 80% of the sites on the internet...<p>I use python, btw, so it's still fairly fast, but for some reason even the simplest pages (one datastore request by key and a simple template) are still not "fast" compared to the rest of the internet.
Here's a very good explanation of why python performance is faster than java on GAE: <a href="http://stackoverflow.com/questions/1085898/choosing-java-vs-python-on-google-app-engine/1087878#1087878" rel="nofollow">http://stackoverflow.com/questions/1085898/choosing-java-vs-...</a>
I was under the impression that Java was faster (even in startup time)...wow. Hope this summary is correct, as I didn't have plans to swap to the java ecosystem.