The screen shakes to let you know you're in the slipstream.
It's not doing that to let you know you will get a tad more speed, it is doing that to let you know you will have reduced front end grip. This is due to the "dirty air" coming off of the car in front. It is doing a lot of work moving air across it and therefore creates a turbulent area behind it. Since you are traveling in that turbulent area the camera (your head) shakes. Your car will have less front grip because the air it is driving into is not a as homogeneous as when there isn't a car in front of you.
The spray coming off the cars in the rain illustrates this dirty air quite well.
If you are using wheel/pedals, there's a bug which disables the slipstream effect. There is a strange fix and that is to set a deadzone in your brake pedal to 2% or more and ... hey presto! Slipstream works.
I always use brake deadzone in racing games so I don't accidentally drag the brake. I had no idea that this was a workaround for the slipstream effect, good to know!