Maybe your site got hacked. 1 - Turn off all plugins, revert to Twenty-Seventeen theme and check site speed. If it still persists, then there is something wrong with your server, SQL DB or DNS. 2 - If speed is OK in step 1, turn your theme back on. If the site becomes slow again, then the problem is something in your theme isnt working well. Install WP-Debug and see what scripts/php process or hanging up your server. 3 - If speed is OK in step 2, turn your plugins back on 1 by 1 until your site slows down. Thats pretty much all I can say without actually taking a look at the site.