A tire only has a certain amount of grip to give, the easiest way to think of it is if you load the tire with either acceleration or braking you are taking away sidebite. Likewise in a corner when the tire is giving all it's grip in sidebite to change the direction of the car it loses forward drive/braking ability.
In general a tire has most grip with a little bit of sliding going on, and thats the trick to going quickly. It's very easy to either under drive and have the car on rails or over drive and not use the tires grip efficiently.
Braking to bring the back end round is a bad thing for going quickly in my experience unless you have a
really tight corner to get round. A slide initiated by locking the rear wheels up scrubs speed and is slow, a slight four wheel drift out of a corner which was started with the throttle is fast
