So, you have a great website. What does it look like on a smartphone? Are the links, buttons, switches, widgets, gadgets, and images all easy to click? Does it take too long to load on a smartphone? Can you read the text or do you have to pinch-zoom to read the title?
Okay, nevermind all of that. Just do this: Go to https://www.google.com/webmasters/tools/mobile-friendly/ and enter in your website.
If you pass, go on over to: https://developers.google.com/speed/pagespeed/insights/
First thing: Get a responsive website.
Second: Get caching in place. We use WP Super Cache and CloudFlare. For our special websites we also use Varnish.
Third: Try to compile all of your CSS into one minified file and all of your JS into one minified file. You may need us or another developer. Another choice is to minify the individual files and use CDN (content distribution networks). CDN allows you to download from more places than just your website. A typical browser can only download four things at a time from one URL and most websites have to load quite a few files. So a CDN makes it to where you can download from cdn1.yourdomain.com, cdn2.yourdomain.com, etc, and the visiting web browser can download four items from each of those places.
A little problem with CDN and SSL – not all CDN have SSL or secure hosting, or can’t allow you to use a subdomain like cdn1.yourdomain.com, you may have to use their special domain for SSL — https://yourdomain.buckets.io.provider.com or the like. This means you can only have one extra place to download. Takes some research to see what’s possible.