edit: interestingly, taking the output priority off the splitters doesn't make much difference.
I use a full counter to check rates, starting the clock and supply counter when the first plate hits the right chest and stopping both the clock and the feed when the 4000th hits it. I show ~5833 plates/min sustained loading for the reddit-post version, ~6469 plates/min for the self-clocking one with priority, here's my full test setup:
pic
Toggle the constant to start the supply belt, you can toggle it back after the clock starts and it'll auto-shutoff the belts when done but the counter stops anyway.