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PHP 7.0 was mainly faster because the internal variable data structure (zval) was redefined such that PHP doesn't need to follow so many layers of indirection and doesn't need a full ref-counted heap allocated data structure for every int and bool in your script.

I believe there were attempts at implementing JIT in PHP 7 which were unsuccessful against the timeline for release. Instead, they did other optimizations and managed to squeeze quite a bit of performance doing this (comparable to HHVM).

I think 7.1 or 7.2 was where it generally beat HHVM.

at least some of the speed bump in 7 came from reduced memory usage. IIRC, array pointer size was reduced significantly - something like 120+ bytes per array entry was reduced to ~40 bytes, so far less memory needed to be allocated and managed throughout an execution cycle. I'm sure there were other reasons for speed boosts, but that is one that's stuck in my head for some time.

PHP 7 didn't introduce a JIT. There was an experiment in the 7.x series, but it never surfaced.