In my experience each new iPhone initially has somewhat worse performance cellular wise than the previous iPhone. I believe it takes time for apple to optimize new hardware via the modem and carrier firmware. Which of course also involves the other parties so it doesn’t happen overnight. I especially remember the iPhone 12 Pro. It took a whole year to get that thing running as it should. It was quite frustrating. I historically do a lot of speed tests for various reasons so I know on average what looks right and when a phone is not hitting the mark. Food for thought, I do think overtime the 17s will perform very well.