As for the optimization so the most important points are the cache plugin (we prefer WP Rocket), optimize all images, use CDN and don’t have too many plugins that may slow your website down. You can try to disable them one by one to check the performance with and without particular plugins. If you will have any additional questions, contact ask and we will do our best to help you.
If you will use deferred JS option you need to be sure that jQuery library is in the exception list. Also, it may be incompatible with additional plugins.