How fast do you need to get to the ball? My answer is as SLOW as possible.
How fast should you get back to the T? My answer is as FAST as possible.
Let’s look at the first one. How can it be possible to say as slow as possible? Well, the idea is that you never want to get to the ball too early. You will waste energy getting there without any benefit to you. In some situations you might be able to play the ball earlier but that’s not always preferable. The idea is to get there when you NEED to but not before.
With regard to the second point, the faster you are back on the T or the right place for the situation, the less options your opponent has.
Most club players do it the other way around: get to the ball as fast as possible and then move back slowly to the T.
Watch a few professional matches on the Internet and see how they move. You will be surprised at how quickly they get back to the T.
Try it for yourself. You will probably find it hard to move to the ball at the right speed and quite hard to rush back, but it’s worth it.