Slow is annoying.
People like things fast. Fast cars, fast good, and fast downloads. Who wouldn't? There's nothing more annoying than sitting on your phone and waiting for something to load. And if something takes too long to load, we move on. That's why performance of a website is so important.
We know from surveys that people expect pages to load in two seconds or less. That's our definition of fast because it's what our users expect. We also know that nearly half of people say they'll abandon a page that takes longer than four seconds to load. In actually testing we know they're a little more patient than that, but if a page takes longer than five seconds, we can expect about half of people to abandon the page.
For that reason we need to consider performance as part of design when we create anything for the web. The reasoning behind it is simple: the faster our page loads, the more people that view our site, and the happier they'll be. More happy users means more conversions.
There are three approaches one can take to reducing the page load on the site. The most direct is to reduce the file size which we can do by stripping out content, or by compressing existing content via minification or gzip. First ask, "Do we really need this?" and then ask "How do we make what's left smaller?" Editing and then compressing content is fairly straight forward because we're just making it smaller.
These are all things an experienced developer should be doing to begin with. Websites that load fast aren't just best practice, they're fun. Tweaking a page to reduce its load time is like playing golf. You want get your score as low as possible through careful optimization of techniques.