Well that depends on what your application is doing. But yes, if it is a database heavy workload, then what you say applies.

What I was trying to get at was that in these cases the ruby/python/whatever parts are not actually fast, they just do much less stuff. Add that the database might be on another machine, so there's plenty of overhead as well.

Aside: You can sometimes get orders of magnitude of speed gains just by using an embedded database and a fast language on top if your workload allows it. Essentially you don't really do much to get this, just cut overhead.