For Play, you'll want to either 1) handle the (blocking) database queries using Futures/Actors that use a separate thread pool (this might be easier to do in Scala) or 2) bump the default thread pool sizes considerably. The default configuration is optimized purely for non-blocking I/O.

See e.g. https://github.com/playframework/Play20/blob/master/document... and https://gist.github.com/guillaumebort/2973705 for more info.