One of the common phrases you hear a lot around the racing classroom is slow in fast out. This is opposed to a more circular arc with constant corner radius and constant speed. There are a couple important reasons why the typical late apex racing line is intentionally slower on the entry than the exit.
Speed – In most racing cars, the engine has enough power that you want to straighten the car out and get to full throttle as soon as possible. The situation would be different if we were racing bicycles, but we’re not. By doing the majority of the slowing and turning early in a corner, you can be on throttle for longer. The more powerful the car, the more important it is to get the slow parts over with early in the corner.
Safety – Cornering has inherent risks, especially since track conditions change throughout a race. The higher the speed, the greater the risk. You can increase your safety margin by cornering at a slower speed. Entering a corner slower also gives you more time to deal with unexpected problems, such as debris on track.
Despite the advantages of slow-in-fast-out, there are a surprising number of drivers who do it bass-ackwards. That is, they go in fast and come out slow. If you have to hit the brakes, release the throttle, or wind in more steering at the exit, you lost some time. If you’re not careful, you can also lose the car.