If you know PHP and CodeIgniter well, and don't know Python very well, then it will take it longer to write in Python.
If you move to something like Tornado (python), Twisted (python) or node.js (javascript) then you might be able to get more performance out of your app, but really only if think moving to that level of complexity is necessary for you app.
In general, it's best to go with what you know. Yahoo! uses a lot of PHP as does Facebook. Since you mentioned CodeIgniter specifically, you're already well down the path of using a framework that allows you to write modular and reusable code with PHP.
If you move to something like Tornado (python), Twisted (python) or node.js (javascript) then you might be able to get more performance out of your app, but really only if think moving to that level of complexity is necessary for you app.
In general, it's best to go with what you know. Yahoo! uses a lot of PHP as does Facebook. Since you mentioned CodeIgniter specifically, you're already well down the path of using a framework that allows you to write modular and reusable code with PHP.