Or, from the article
Apple's challenge is that few of its current models are high-tier devices when we group iPhone models based on their mobile network experience capabilities. In our measurements, just 14% of Apple users are high tier. Instead, Apple has chosen to focus its handset designs on other capabilities such as facial recognition, camera innovation, long battery life, and extremely fast application processors and graphics using Apple's in-house silicon designs.
So it seems that even in countries with excellent infrastructure, Apple seems to be unable to capitalize on it. It's probably a relatively irrelevant metric since Apple has prioritized how the phone is used over absolute speed. That's a very Apple thing to do.