In my offline races it seems that AI is faster when it has to follow, or recover from behind (maybe after an incident)... instead cars in front doesn't do their best it's just my sensation?
I haven't experienced this. Once I pass a car they generally don't get back by me unless I start making mistakes (which is not uncommon). If the leader is just flat out faster than everyone and pulls away, they generally stay away in my experience. Now, there does tend to be some evolution during the race where some cars get slower and others get relatively faster (as happens in real life), but that seems to be essentially random as opposed to being based on where the player is running.
People say that AI gets a bit slower when they are in a pack, so if one car spun out and is now driving alone, it might be faster. Then, sometimes they try to defend their position from you, this will also make them a bit slower.