I'm not sure if they are purposely throttling but they definitely aren't living up to their speed/coverage claims.
I'm absolutely DONE with AT&T.
A few weeks ago, I called and told them that data wasn't working at/around my workplace (a high school) and that I had spoken with many other AT&T customers (students/teachers) and all of them reported having issues.
We did 2 hours of troubleshooting to eventually figure out it was specifically the LTE signal from "our" tower. I was told they were sending an advanced technician to the location this was happening (the school) and the tower and that they would be in contact with me within 2 weeks (but tech rep on the phone said they usually give you a call after a few days and she said they had him scheduled for the very next day). Their solution for the issue was to "disable LTE data for now." Ok...so did that, and spread the news to the kids/teachers and most did the same, awaiting a fix.
Anyways--fast forward two weeks, they give me no update--no phone call, no email, no text...nothing. Even after having an "official case number" and "an advanced technician" assigned to my case. So, I went and switched. Was pain free, and I was in and out in under 30 minutes with unlimited data for only $15 more than I was paying for 6GB (of 4G since they didn't feel like fixing their LTE apparently). I have great LTE service everywhere I go now, and I haven't had any issues with things not loading or messages not sending. It's now been over a month, and my AT&T having students are STILL having to use 4G. I encouraged them to call and open a case as well--but who knows?
Sorry to rant. I hope your data speeds get better.