First I would visit a single file in the browser, like an image file, to see if the server loads that quickly or not. If the image loads slowly, you will know it is a server speed problem. But if the image is fine, most likely there will be a PHP code slowing the website down, coming from a plugin or the theme.
You could use Health Check & Troubleshooting plugin to try to find the problematic plugin/theme quickly. Go to the Plugins list and click on “Troubleshoot” at one of the plugins. This turns off all other plugins and switches to a default WordPress Twenty[X] theme if there is one available, but only for your user, so the real website stays as it was. On the Plugins list you can turn on plugins one-by-one (or a couple together) with “Enable while troubleshooting”, and this way you could see if there is a plugin involved in this matter. If the plugins are fine, at the top admin bar within “Troubleshooting Mode” you can select the original theme in “Themes”, to see if that makes the website slow.