Research from Twin Prime Inc. and Cellular Insights recently found iPhone 7 units with Qualcomm modems aren't seeing the kind of data performance that should exist. Sources close to Apple told Recode that the discrepancy is due to a performance limit placed on Qualcomm's component.
The iPhone 7 uses modems from Qualcomm when unlocked or tied to Verizon/Sprint. Whereas it uses Intel modems when locked to T-Mobile and AT&T. For the sake of standardizing performance, Apple purposefully reduced the speeds offered by Qualcomm's X12 modem.
Units with the X12 still perform a little better than those with Intel hardware, but the performance difference isn't as great as it could be.
This performance limitation shouldn't have a notable impact on what users perceive.