The network speed is part phone and mostly the cellular network. Anyone that claims it's the phone is suspect if the phone radio is constant.
I am getting ready to buy the wife an I 7+ to replace her Droid Turbo.
I just read this morning that the 32 GB phones are much slower in some ways than the 128GB version. The article states " write performance for the 32 GB iPhone 7 was only 42Mbps while the 128GB was over 8x faster at 341 Mbps.
The article also states the 4G network performance of some 7+ phones is 30 to 75% "worse" than other model numbers.
I thought the beauty of an iPhone was that the performance was consistent.
Now you have to know the model number to get a "fast" $869 iPhone? Maybe we should buy a Samsung S7 or Pixel instead.