There may be many reasons for the speed of the site to decrease on Google. If your site is mobile friendly then you need to pay special attention to it. According to Google algorithm rules, you have to keep updating your site's meta title, meta description, meta keywords. If your site is mobile-friendly, then it has been set different rules in the Google algorithm. You have to keep the on-page of your site regularly user friendly, attractive, and compliant with Google algorithm rule. If it is followed properly then your site will definitely be maintained.
Sometimes errors come in our favor. We must continue to explore it. And should be repaired. This may also be a reason for the speed drop. Using google webmaster we can detect different types of errors. This makes it easier to solve.