Every time a wing generates lift (or a spoiler generates down force) it also generates drag. Drag is the natural reaction of the fluid (air) to resist motion through it (the car). Drag is bad, because it slows down the car. So, more down force is good... but too much down force = too much drag, which is bad. Very very high performance sports cars, like Le Mans or F1, have a ratio called the 'lift/drag' ratio. The car designers try and maximize this so that the car has just enough force to get around the corners, but not so much that they are too slow. Indy cars, and ones that are designed like that can have down force on the order of 3G's, at 200mph. That means they could hang completely upside down on the track, and as long as they kept going fast enough, they would still stick to the road.