ACC is actually the one, or one of a tiny few games, where keeping TC on is faster. Unlike GT Sport, or even AC, where the TC system is not the greatest and can cost you lap time. In ACC, Kunos have implemented it very closely to the real system used in the GT3 cars, so keeping it on 1 or 2 is actually advisable and the quickest way to drive. There's a video where David Perel talks about it and as he is/was a real Blancpain driver who helped develop ACC I figure he knows what he's talking abou.