Your performance concerns (and I believe the other comments have touched on this) likely don't matter for most real world sites, and then, even if you get big, you can probably work it out (Pintrest is done with Django I believe, Github is rails, etc).
The question you really want to ask yourself is what does your compiler/runtime do for you. I've been pretty much solely a Python developer for the past year or so, but I like Java (... yea, I know), and find Go interesting, too. A compiler is no excuse to not write tests, but you know what you've written is going to run after it compiles. Added datetime's wrong in Python by adding one that has a timezone marked and one that doesn't? Runtime error. In fact, everything is a runtime error. Tests should not be optional regardless of language, but with Python and Ruby, they really take the place of the compiler in statically typed languages.
I don't want to discourage you from Python by any means, the Django community is fantastic, and Rails 4 with it's Russian doll caching seems pretty sweet. And nothing can beat the libraries for these two projects. The other options that I've seen (ASP.NET, Java Play, Go Revel, all for which, admittedly, I've only done preliminary reading about) just don't seem to have the libraries to get you off the ground as fast (though, if you've never written all the tedious stuff like user creation, email password resets, it is a good exercise in web development basics).
If you stick with Python I encourage you to: 1. Tackle learning the best practices for testing as you go. 2. Check out Two Scoops of Django [1] after having used Django for a bit. 3. Learn good server management skills (maybe look into Puppet [2])
The third seemingly is an offshoot, and would be necessary for really any framework, but once you're ready to deploy your app, you'll likely spend a great deal of time comparing your various options for stack choices and will get your sever set up just how you want it. Make sure that process is easily repeatable. If you don't like how Puppet does it, look into Chef or Salt Stack or whatever; these will save you tons of time down the line.
[1] https://django.2scoops.org/ [2] http://docs.puppetlabs.com/mcollective/reference/basic/getti...