Jetpack by WordPress.com
Activate site accelerator tools and watch your page load times decrease — Jetpack optimize’s your images and serve them from their own powerful global network, and speed up your mobile site to reduce bandwidth usage.
The Jetpack is the most used plugin for other purposes such as security and can also be useful for improving performance. After installing this plugin, you will need to activate this and the open settings and “Performance” tab for configuring this plugin.
Jetpack plugin is available at https://wordpress.org/plugins/jetpack/
W3 Total Cache
The highest rated and most complete WordPress performance plugin. Dramatically improve the speed and user experience of your site. Add browser, page, object and database caching as well as minify and content delivery network (CDN) to WordPress.
This plugin is available at https://wordpress.org/plugins/w3-total-cache/
Autoptimize makes optimizing your site really easy. It can aggregate, minify and cache scripts and styles, injects CSS in the page head by default but can also inline critical CSS and defer the aggregated full CSS, moves and defers scripts to the footer and minifies HTML.
This plugin is available at https://wordpress.org/plugins/autoptimize/