A race car generating large amounts of downforce is also generating large amounts of drag. The two cannot be separated cleanly, and the balance struck between them defines how a car is set up.
Downforce and drag share a source
A wing works by accelerating air over one surface and slowing it over the other, producing a pressure difference that pushes the car downwards.
Turning the airflow in that way also disturbs it, leaving behind a wake of slower, disordered air. The energy in that wake comes from the car's forward motion.
That energy loss is drag, and it grows as the wing is set to produce more downforce, which is why the two quantities are usually discussed as a ratio.
The benefit appears only in corners
Cornering grip depends on the load pressing the tyres onto the road. Downforce adds load without adding mass, so it raises cornering speed without adding inertia.
The effect grows with the square of speed, meaning it is negligible at low speed and enormous at high speed. Fast corners benefit far more than slow ones.
On a straight there are no lateral forces to support, so the downforce does nothing useful while the drag that accompanies it continues to cost speed.
Circuit layout decides the compromise
A track with long straights and few fast corners rewards low drag, since time lost in the corners is smaller than time gained on the straights.
A circuit of linked medium and high speed corners rewards the opposite, because each corner taken faster compounds into the following straight as well.
Teams therefore arrive with different wing configurations for different venues, and the choice is made before the car has turned a wheel in anger.
Ground effect changed the efficiency of the trade
Shaping the underside of the car to accelerate air beneath it produces downforce with much less drag than a wing generating the same load.
The floor works best when held at a consistent, low ride height, which is why cars using it run stiff suspension and are sensitive to bumps and kerbs.
Because much of the load comes from underneath, these cars lose less performance when following another car closely than wing-dependent designs do.
Adjustable aerodynamics attack the compromise directly
Devices that stall or flatten a wing on the straight reduce drag temporarily, then restore downforce for the braking zone and the corner.
This gives a car much of the benefit of both configurations, which is why such systems are tightly regulated wherever racing series wish to limit performance.
Where they are permitted with restrictions on use, they become a strategic tool as much as an aerodynamic one, since the advantage is granted rather than continuous.