Practice is just the process of gaining experience. It's that experience that allows you to improve.
In other words, you need to gain experience doing the thing you want to improve at, and if you're not actually doing it, you're not experiencing it.
So if you want to improve at look ahead, you need to actually look ahead. That means slow down to a point where you can actually look ahead. However, if you slow down too much to the point of it being trivial for you, then you are only experiencing something you are already able to do. To continue improving you need to practice in such a way that you are experiencing things that are just beyond what you already can do easily. This is just a simple way of describing deliberate practice.
In your case this basically means turn fast enough that you are not looking ahead successfully 100% of the time, but pretty close.
If you turn so fast that you are not able to look ahead at all, then you are not experiencing it, and will not improve. If you turn so slow that you can look ahead without effort, then you are not experiencing something with room for improvement, and will not improve. Design your practice sessions in such a way that you are always in the middle ground between those two.
Worry about TPS when you get to a point where you can't turn faster than you can look ahead.