I have to say I've found WordPress and Automattic lacking in encouraging PHP upgrades, it has been very underwhelming.

Instead of Mike's idea of artificially slowing pages, why not just quantify how slow php is in its current incarnation compared to PHP 7+.

I'd like to see a nag notification on every release saying something like: "X WordPress users have upgraded to PHP 7+ since the last release. PHP 7 runs x times as fast as your current version, learn how to get an upgrade ."