Which is the most efficient way (performance) to add functionality to MediaWiki?

Question

I know of three ways to enhance MediaWiki functionality:

1. Using extensions.
2. Using templates.
3. Changing MediaWiki:Common.css & MediaWiki:Common.js

My question is, when I can choose which to do, which is best for performance? Obviously, templates only affect the pages on which they are included, but are they then faster than .css & .js changes? And which will slow down MediaWiki most - .css / .js changes or new extensions?

Specific example (following jscott's comment). A user wants to hide the numbers in the table of contents:

1. I can use an extension: PSINoTocNum
2. I can use a template (including this code: .tocnumber { display: none !important; } which I can include only on the required pages).
3. I can change MediaWiki:Common.css.

Which has most / least performance hit?

Note - this is just an example. I want to know generally which approach is best and why.

Answer 1

This question won't have a proper answer because rendering changes (css, js) depend upon the visitor's computing power and connection speed whereas MediaWiki extensions primarily depend upon the server's computing power.