Att unlimited premium has slow data speeds.

Why is my data always slow and inconsistent ? I pay $85 monthly for a plan that states the data will never slow down. 

That is NOT what it says.

It says:

Unlimited talk, text, & high-speed data that can’t slow down based on how much you use

That is quite different than what you are saying. 

Also every other plan says speeds can be reduced due to network congestion  This plan mentions nothing about that  

Also every other plan says speeds can be reduced due to network congestion  This plan mentions nothing about that  

The top of https://www.att.com/plans/unlimited-data-plans/ says:

Scroll down another few lines and it says:

From left to right is Premium PL, Extra EL, and Starter SL.

• Starter SL is the lowest priority.
• Extra EL is higher priority, (still affected by congestion though), but after 75GB it becomes the same priority of Starter EL. Basically, a skip-the-line pass when compared to Starter SL, but after 75GB, they lose the pass.
• Premium PL is same priority as Extra EL, but it never lowers the priority. But it still affected by congestion though (a traffic jam is a jam). Premium also has a skip-the-line pass for Starter SL and when Extra EL exceeds 75GB.