On a reported Intel i3 3240 – 8GB ram, from version 7.0 through to the latest dev build of 7.4 the speed improvement is decent.
By speed I mean the calculation and processing time PHP takes to solve in the benchmark. For a so-called dead language PHP 7.4 looks to be very snappy.
Whilst improvement through updates is pretty much a given in software development, for an aged language like PHP to still see vast improvements is satisfying.
course like most things computing the end result will be determined on your hardware. Faster CPU and ram will see improvements or lesser times in the benchmarks.
Perhaps by stating the obvious, updating or upgrading your PHP version on your web server is worth it. From the security to performance and speed improvements.