Hello,
These reports grades just show that your website content should be optimized better but don’t mean that it is slow. So, all you need to do now is to follow all tips from GTMetrix and optimize all your images, configure cache and server GZIP compression. Here are a few articles that should help you
https://gtmetrix.com/wordpress-optimization-guide.html
http://www.wpbeginner.com/wordpress-performance-speed/
https://premium.wpmudev.org/blog/w3-total-cache-settings/
Write us if you will have any difficulties with configuring of the W3 Total Cache plugin and we will help you. Also, contact your hosting providers to turn on GZIP and optimize all your images.
Regards