Well, if your database is the issue 2 things I would do:
1. Switch to MariaDB, you should get slightly better performance. Switch is relatively simple for a server admin who knows how to do it.
2. Consider implementing Memcached to cache database queries.
You should also consider APC or xCache to cache PHP.
On the other hand, consider W3TC or WP Rocket for caching inside WP. They are much better than WP Fastest Cache.
Since you're using Cloudflare, you wouldn't need another CDN.
If you're using related posts plugin, they tend to be resource hungry. Consider switching or removing it.