Slow websites are lame, and Google knows it. Starting in July 2018, Google will use a page's speed in the ranking algorithm for mobile searches. That means slow websites will be penalized, moving faster ones up closer to the top.
Google calls this the "Speed Update," which is not a terribly imaginative name if we're being honest. Only the slowest pages will be affected by this update, and the same standards will be applied to all sites regardless of the technology used to build them. Slow pages won't be completely banished, either. A slow site that ranks highly in other ways could still show up near the top of your results.
Searches on desktop have used page speed as one of many signals for several years, so this move is not without precedent. Developers already have tools at their disposal to test their sites to make sure they won't be penalized. So, you might see some mobile sites speed up as we approach the Speed Update.
Google has confirmed that the Speed Update is rolling out to all users as promised. Your mobile searches should begin incorporating speed ratings soon.