After been able to scale the duration of each race in championship I decide to give it a go with 10 minutes races. The first track was ok, the Ai a bit weak but not by much. Then in the second track the AI was at a turtle peace. To the point that i was able to lap the whole grid making mistakes and without knowing the track. Here a video from my session in the second track, the Ai behaviour can be seen from qualify to the two races sessions. Please fix the Ai, in the long run it's what's keep the game alive in the library of those that do no like online racing.
I noticed this too, I think the Championship mode is not ready for prime time. The AI strength in the first race is determined by whatever you last used in single race, I confirmed this (you have to restart the series after changing the AI). But in subsequent races it must be defaulting to a very low value. The good news is I don't think it's an AI problem, just a Championship mode problem. You should be able to do each race in single race mode without this issue.
