PHP 7.2 was released late last year, and is now recommended by https://wordpress.org/about/requirements/.
75% WordPress sites are still using PHP 5.x according to https://wordpress.org/about/stats/
I just thought I’d make a comparison of performance of a few recent versions of PHP.
This is a comparison of the WordPress page load speed. This isn’t the whole page loading in the browser, it’s just the php dynamic portion. Whilst this is just against 1 site, the relative performance will be indicative regardless of site design , templates and images.
If you aren’t sure what version of PHP that you are using, try my plugin, it will tell you : https://wordpress.org/plugins/wp-hosting-performance-check/
I wasn’t able to get timing on older php versions, but from experience they 5.5 would be about double the time of 5.6, and 5.4 would be 4x that of 5.6.
The outcome is clearly you need to be using at least 7.0, and 7.2 is considerably faster again.
I think the 7.0 to 7.1 might just be an anomaly, or a different php module being used. I am not sure if the difference is real, in that 7.1 is slower for WordPress or not. But PHP 7.2 is definitely faster.