Slow websites are the death of SEO. Honestly, you can do everything else right and if your website is slow it wont rank. Google LOVES LOVES LOVES fast websites. In Google’s eyes a fast website equates to a good user experience. Google prioritizes user experience.
No matter how good your content is, if your site is slow no one will ever read it!!!
Here is a free tool to help you measure your site performance. https://gtmetrix.com/