We will work through this from the bottom up. This is not a priority order, more detailed posts will develop from each subject. More links and content will be added in time.
Your host needs to provide a good server. SSD is almost expected in 2017. They will often have added hardware for Cache and faster content delivery. You also need to know that it will not be crammed with hundreds of other sites all fighting for processing space.
You should be using PHP7. it is faster than older versions and take advantage of the latest chip technologies for quicker processing. Owners have reported 20-40% speed increases from older PHP versions. You hosting company should be providing a PHP7 upgrade for you in 2017.
WordPress and Plugins
Must all be the latest versions to avoid bugs and gain the best speed advantages. The plugins need to be well written. Such nuances may only become apparent on larger sites, but I have come across pitfalls where programmers have not though issues through and written inefficient code.
Try the P3 Profile plugin to catch slow plugins and then ask them why.
Your theme delivers the content so it is the highest priority for efficiency. This is when all those plugins popups and share buttons and animations can suddenly be bad news for delivering the fastest site you can.
Compression and Minify
You can make a site faster by reducing the amount of data needed to show it. That means compressed code (often called minifying) and optimised images.
Cache and CDN
Cache software will keep copies of pages and then deliver them to the next visitor without going through all the process for that page again. This most improves as the site become busier. CDN will deliver static data like images on faster servers often physically closer to where the visitor is.
Mobile are often on slower connections even with 4g. They have smaller screens which can show less information or data. You need a mobile strategy and even look at the new technologies to deliver a better experience for mobile consumption.