I use Ruby a lot, and I always deploy Ruby web apps to Heroku. This is all about loading request times. Right now, I think that the best bet for AppEngin is coding in Python. Since I don't much use Python I have been experimenting with using Java + Objectify instead of JDO: this gets my request loding times down to usually just over 1 second. I have two writeups about this on my blog.

I have spent quite a lot of time trying JRuby + Sinatra + AppEngine. Lots of good work but not really ready. Google needs a paid for option to always keep at least one JVM for an application spun-up.