Game lagging

When I’m in a very close race, my game starts to lag. This has happened to me multiple times and is very frustrating, I finally catch up to the lower class car in online racing and then my game lags and they instantly jump 5 cars ahead. What is the cause of this and does anyone else experience this problem. It doesn’t happen if have a big lead, but the instant I have to catch someone and finally do…lag. This hasn’t happened to me on any other game except Horizon 2. Forza 6 is perfectly fine so I know it’s not my internet.