When a users searches on google or another search engine and your content shows up for a search they did and they land on your website and it loads too slow then the website visitor might give up on your website loading and click the back button and go visit another website.
Google wants to show results from websites that will give a searcher a good user experience and a slow loading website is not a good experience.
You should aim to have your website load as fast as possible to get good rankings and to help lower the bounce rate.