Losing speed on straights

I recently bought the game and it is the first sim racing I’ve played in a very long time, so all assists are on and AI difficulty is set to 20. But I always lose speed on straights, and other cars catch up quickly, even without the DRS. For now, I try to follow the race line and learn the tracks but no matter how well I exit the corner, other cars catch up to me, I’ve tried using setups in the game and off the internet, lowering downforce to no avail. Does anyone know how to fix this?