Just another reference: iPhone 15 Pro, 512 MB memory, running IOS 17.2.1, running a four APs meshed ( Arknis 810 ) in my home, connected to a Fidium 1 Gb Fiber ISP, using an Edge/ UI router.
For throughput testing used both olkla (Speedtest) and fast.com.
Speedtest - 590 D and 588 U
fast.com - 798 D and 799 U
its always good to check which radio you are using WiFi or Cellular - with the fast cellular speeds - it might be that your not really connected on WiFi. My AT&T bandwidth is amazingly good with 5G - I’m grandfather to a no throttling plan which helps.
I recently upgraded to the new Arknis AP from the 500 model, which made a huge difference in performance. It was like my new iPhone resource depleted the APs hence giving me poor throughput. I understand the protocol over WiFi and it’s possible to overwhelm the older APs…
in my limited testing, with multiple WiFi environments, the iPhone 15 performance is amazingly fast with a modern WiFi APs
i hope you find the root cause of your performance issue.