It's helpful to separate performance from the ability to perform. It's generally not the greatest idea to prematurely optimize, but you should absolutely be making decisions that give you the flexibility to optimize in the future. For instance, adding a caching layer before it's needed may be a bad idea, but organizing your data in such a way that you could easily add a caching layer may be a good idea. Making your website in C because it's fast may be a bad idea, but choosing Python because you could easily add C bindings to the slow parts may be a good idea.