Unused RAM is wasted RAM. So not really Apple magic, about every OS uses "free" memory as disk cache.

Try to leave only a gigabyte or two free, speed likely would drop dramatically.

Edit: or do some calculation / logging of experts read speed, to see if it's faster than SSD spec.

yeah, looks like a page cache matters a lot I tested on mine m5 pro with 8gb memory pressure, got 27t/s instead of 35t/s Someone tested on m4 max. In regular state it was 48tok/s, but 32-42 with memory pressure