In order for your web pages to be correctly indexed on search engines, it is essential that you facilitate access to them for both users and robots. To do so, an array of criteria is considered, each one weighing in more or less heavily in the web indexing balance.
For example, a site with a low page speed will tend to enjoy poorer web indexing than a faster version. Indeed, on faster sites, robots will have more time to visit more pages, hence indexing on all content. Similarly, users who have to wait to see a search result load will generally be quicker to leave the site. The resulting high bounce rate will tend to weaken the quality of contents on the page in question.
Note that these technical criteria regularly evolve based on changing Google requirements. For example, although of lesser importance a few years ago, moving to the HTTPS protocol has become a priority.