Racing Ginettas at any track destroys fps I get

I started a 20 car Race with the Ginetta junior at Donnington and I couldn't get over 40 fps tried again with a mx5 cup and was getting around 90fps, I tried a kunos track,brands hatch, and still had the same issue, it's not the first mod that's done this to me just frustrating as it feels sickly to drive with such low fps, any ideas would be great