So if you wanted to confirm the data path only using a different app with same source (no chances for transcoding or other fiddling), could I suggest that you install Infuse (free version is fine) and run some tests.
For me Infuse on iOS (iPad Pro) is typically at least 60-70% faster (when viewing from Plex console dashboard). This will tell you if it’s something to do with Plex client OR iOS/Wi-Fi/Network/Plex Server/Plex Server Hardware.
In my case, Infuse is always significantly faster (at least double) every single time that I test. Other’s have confirmed this also over the years.
Here is a thread with screen shots from 2022!