Up until now Google has only concentrated on the speed of desktop searches only. From July 2018, Google is set to do an update called “the Speed Update”.
Google has said that this will only affect a small percentage of searches and that it will only be those pages in searches that deliver the slowest experience to users that will be affected by the update.
In order to check the speed of your website, you can use this Page Speed Insight tool to measure speed and make improvements. (Size and resolution of your images on your pages play a major part in the speed of your website).