Nobody likes sites that are loading for eternity. Page Speed Performance is one of the most important SEO metrics.
Consider this: site speed is the first impression you ever make! And first impression is vital for people to stay and read your content
Also, ensure the site is mobile friendly, preferably with a responsive design to any device.
A reliable tool that you can use is PageSpeed Insights from Google.
But I also recommend Pingdom Tools, as it will give you the advantage of changing the server’s location and check the site speed from different regions (eg. North America, South America, London, Australia etc.)