Provide sharding, hashing users to the same R/W master, db pooling etc etc. All the stuff you generally have to write for yourself when a site needs to scale.

Symfony is the same as any other framework, if you can’t do it you have to write it yourself.

I liked his graph comparing writing yourself and writing with a framework where the time plummets when you hit against something the framework “can’t do”.

When you are developing you need to make sure that you don’t do this, because if the framework can’t do it…not really a problem as the frame work is written in your language of choice it means you can just code your solution to fit into your framework.

Sometimes this means you could just write a plugin, for example what we did for Symfony to use XSLT as our templating language.

Also some of the issues that can slow things down is confusing the “M” in MVC to be the objects provided by the ORM.

You should model the “M”s you need…if they happen to fit exactly the “M”s provided by the ORM…great job done…but sometimes in complex applications that are not just CRUD they don’t. If your in this mindset before you code with your framework of choice you won’t hit against the dip as much.