My company did some email marketing that would spike traffic to 50,000 visitors in 30-60 mins and Wordpress MySQL would go nuts. Who knows how many horrible plugins marketing had in there but first go to for me was varnish, then cloud flare then final awesome fix was swapping everything to static HTML that we crawled every night ourselves from our own htaccess blocked Wordpress. Only thing that had to be dynamic /do related was the contact form.