Jani Tarvainen explains where PHP is heading performance-wise.
PHP 7.0 was a leap in performance that came with very easy adoption. Simply verify compatibility with the version and upgrade your server environment. Speeding up many older architecture apps like WordPress and Mediawiki by a factor of two is a testament to backwards compatibility.
In 7.1, the language runtime will continue to make modest improvements, but bigger gains will have to wait. One of these opportunities for a bigger improvement is the JIT implementation that is now bound for PHP 8.0