without getting over technical walrus, because I couldn't even if I tried
usb kb's or basically all new kbs are digital, meaning when you press a key it sends a digital signal through a chip this gives a delay
mechanical is old school, remember the 90s IBM white keyboards that were everywere? there's no delay to the keys, it uses a more rudimentary system based on switches that send analog signal. also, the keys have that old feel, they CLACK when you press them and they move up super fast allowing for faster taps. It has more of a steel feel.
in resume, mechanical keyboards are extremely faster than keyboards you find nowadays in shops. you can basically obtain FR taps with one finger, providing you have very good timing. and if you FR well, imagine fingerolling super fast on this kb's spacebar, to the eye it would seem as if the space bar never goes down, because it goes back up THAT fast