But downforce isn't appreciable until over 50mph (80kph), short of having stupidly oversized wings (which would drastically slow your acceleration) or active downforce (fans etc).
This is actually why a lot of expensive sports cars have 0-60 times of 2.9 seconds. Unless you design specifically for it, it's difficult to generate much downforce at low speeds.
Related: if you've ever wondered why some drag cars (e.g. Funny cars) have short exhaust pipes angled up and back, now you know.