realpath_cache_size, on a bind-mounted volume

Every file resolution is a stat, and on a bind mount each one crosses a filesystem boundary — which is why this setting matters most where it is least often set.

realpath_cache_size=4096k    ; default 4096 entries
realpath_cache_ttl=600       ; default 120

; and the measurement
; php -r 'echo count(realpath_cache_get());'
; 8412        ← against a 4096-entry default

On macOS with a bind-mounted project this is the single highest-yield PHP setting available, frequently worth thirty per cent of the request. In production with opcache and validation disabled it matters much less, which is why it is usually configured in exactly the environment where it does the least good.