I would to add that I have been dealing with this same issue as well. Although it's not throttling, it's the ATT network based on my testing. The AT&T network isn't delivering more than 5 mbits on average. I ported over to tmobile today and all my problems went away. Getting about 76 mbits in Miami with T-Mobile. I attached some screenshots from speed tests on AT&T taken a few days ago.
My testing involved driving to different points of the city and doing speed tests with an AT&T 6s+. This is either a network congestion problem or AT&T has failed to grow with the times. Either way, I switched, I am saving money and I have about 10 times the speed I had before.
To my knowledge, I have been having the problem mentioned on this thread for a good 12 months, I do not think this is limited to just apple devices.
edit - I have attached a third speed test from a oneplus one android phone to further demonstrate that AT&T is just not doing so well with their service.