Speed, quantity of data, and priority are three different things.
If you’ve got turbo then you’ve got higher priority, and if there’s a long line of people waiting to ask AT&T for some data (congestion), you get to skip the line and get your data served to you sooner.
Speed is basically determined by how much data that tower can hand out at that particular moment. I think of it as if you’re doing a speed test, you’re grabbing lots of blocks of data, if you’re higher priority, you keep cutting the line so you’re getting your blocks of data sooner, which makes your speed appear faster as you’re not waiting for the other people who are in line.
To be clear, you’re not really throttled at 75 GB, your priority goes down. If there’s no congestion where you are you’re not going to notice when you get your 76th or 77th GB of data. But if there is congestion, you’re not gonna get served till after the people who are using turbo, after the people who are on premium, And after the people who are using extra, but didn’t use up their 75 GB yet.
The only actual throttling you get is when you use up all of your hotspot, then you’re throttled to 128Kbps (which is pretty unusable, so I just tell people they’re out when they hit that number).