Must be...I know for a fact that some of the license test times benefit greatly from sliding through corners.
The reason on the license tests that sliding works so good is because SRF(skid recovery force) is on. The SRF function is despised by many racers on this forum. It changes the physics of the game, and in my view makes it more like an arcade racer. For some reason it is forced on for the license tests and the last set of bonus races. No one can seem to figure out the logic behind this.
My best answer to the OP about sliding and overall speed. It really depends on the following corners/straights. You will almost always have a lot more exit speed by grip driving then drifting. You might get through the initial part of the turn faster while drifting, but will be giving up massive amounts of time on the straight that follows this turn. I try to only drift the car slightly on the initial phases of a sharper turn. (to get it pointed toward the exit or apex of the turn faster) But I try to be under grip upon the end stages of the turn, because I want to be able to apply exit power as soon as possible.
Hope this helps
Edit: In reply to the post above. If you miss the apex of the turn and need braking/and or change of nose position of the vehicle, then yes, I sometimes slide the car. This is only to try and get back to the racing line as quickly as possible. But this is corrective of a mistake, and is obviously slower than taking the appropriate line from the start. It is quicker than crashing off the track.
I guess to better give an explanation of when I try to drift and when I don't. If I think I can increase my entry speed, without effecting my exit speed (and ability to exit with power), then yes I will slide. This happens on slow, tight, 180 degree turn(can be less or more degrees, really depends on the corner). Or if I can use the slide to accomplish two things at once. Using the slide to help slow the car to the neccessary speed while also getting the nose of the car pointed in the direction I need for corner entry/apex/exit. Even with these examples, it doesn't provide that much help, and when in doubt, I would definitely choose grip.