Google uses speed (and other criteria) when ranking a website. The faster a site loads, the better. You can test sites at Google Page Speed Insights. It tests Mobile as well as the Desktop site, as Google considers Mobile at least as important as Desktop.
This blog and my business site score a high 98 for Desktop and quite acceptable 76 for Mobile. They have separate themes for Mobile. Your site should too. Don’t rely on the Desktop theme for Mobile as it may be slow. Users will leave sites that load slowly.