Considering the performance of your responsive web designs is as important as the aesthetics. However, performance presents something of a moving target. For example, browsers update and improve the way they handle assets, new techniques are discovered that supersede existing 'best practices', technologies eventually get enough browser support that they become viable for widespread adoption. The list goes on.
There are however, some basic implementation details that are pretty solid (well, until HTTP2 is common place, more of which shortly). These are:
Minimize the number of assets (for example, don't load 15 JavaScript files if you concatenate them into one).
Minimize the page weight (if you can compress images to a fraction of their original size you should).
Defer non-essential assets (if you can put off loading CSS and JavaScript until the page has rendered it can greatly increase the perceived load time).
Ensure the page is usable as soon as possible (usually a by-product of...