Website speed is an important one, that you should consider. Your website speed is one of the important factor influencing your Website ranking. And also when your website is slow, Your visitors may not visit you next time. Thus it judges the traffic to your site.
Caching is the most important one. This will increase your speed to a greater extent. Use caching plugin like auto-optimize https://wordpress.org/plugins/autoptimize/ to enable cache functionality.
You can increase the speed by decreasing the memory size of your resources. Important thing is that you have to compress the images. For that you can use plugin called
"Smush Image Compression and Optimization" https://wordpress.org/plugins/wp-smu**bleep**/
Minifying html,Css,js reduces resources size, Thus increasing speed. You have to minify html, js, css. For that plenty of plugins are available. You can use https://wordpress.org/plugins/minify-html-markup/
4.Optimize Css delivery
Optimizing Css delivery will increase the loading speed. You can use a plugin called "Speed Up – Optimize CSS Delivery" for optimising your css delivery. https://wordpress.org/plugins/speed-up-optimize-css-delivery/
5.Optimising Youtube video delivery
When you are displaying youtube videos in your page use "WP YouTube Lyte" https://wordpress.org/plugins/wp-youtube-lyte/