What are some specific things to avoid when writing a theme, to ensure maximum performance?

Lots of threads recently have been shitting on Magento 2’s performance and in each of the threads there are comments saying that a badly-written theme by shitty developers can make or break a website.

But none of the comments actually go into detail about what makes a bad theme so bad, and I’ve not seen a guide about how to write a good, performant theme.

So what are some mistakes which these “bad developers” make which affect performance so much? And how can a bad developer turn into a good developer?

TL;DR How make Magento 2 go fast?