That's because it almost completely depends on the speed of your card.
Look
here for options and comparative speeds.
Also, I'd ignore the peak speeds, as well as the read times - when you're waiting for a thirty-shot action burst to save to card, it's the average write speed you're interested in, not the maximum or read speeds.
Sony really should have put two UHS-II slots in the A7r3 and A9, instead of one UHS-II and one UHS-1 - the camera can only write at the speed of the slower card, which really slows you down if you want to use dual cards for data redundancy.