Solved

Is it real?

https://ibb.co/cfvoBJ

Today one of my competitor win fair and square. But what bother me is he use booster on the last lap and produce a lightning lap time. It is so unreal .. 
What do u think? 
My suggestion is, booster can only be applied during acceleration. Braking will turn off booster automatically, and while cornering, booster will have less effect than on the straight.

This is how boost is supposed to work, it makes you go quicker. He probably has a higher level of kers & drs which made it look unnatural. Ofcourse this is unrealistic, boosts in most racing games tend to be unrealistically powerful. But if you still think this person was going to fast put some more information about what happened, images and HQ levels would help too.

Yeah... 4,8 seconds faster than his average lap time...

This is normal when you save a huge portion of your boost to use on the last lap. You may even have a shot at setting a lap record if you do this in 100% distance leagues, where the more laps there are in a race, the further the lap times will fall as the race goes on.

Ok so if used the way you want it to be and that driver gains 0.2 seconds a lap what’s the difference between using it all on 1 lap and gaining 4 seconds or using it to gain 0.2 seconds a lap for 20 laps 
They have still gained the same amount of time in the race