The cars get gradually faster (but wear more tyres) as you increase the stiffness, the default setups are soft and very generic so you would expect a proper setup to make you a lot faster.
As it turns out the biggest gains are from setting the suspension to the hardest setting, which appears to give the car more stability (though you can certaintly still spin in career mode) but does wear the tyres a lot faster, the result is more responsive and takes away understeer issues meaning you can brake a little later and still hit the apex, allowing you to get on the throttle a little earlier, I personally I've been gaining around 2 seconds from it in qualifying, I had already got my first podium in career while using the default setups, to me the difference is not as big as people seem to make out.
Why do people think DRS through eu rouge is unrealistic? As I already said, they banned it this year in real life because they knew drivers would use it there despite the obvious dangers.
For clarification
Suspension stiffness default is 6 (of 11), the higher the setting the more stable the car becomes and understeer is removed, so it isnt a magic 11 is awesome and 10 is useless, 10 is just slightly worse than 11. The fact that the cars are universally faster this way is probably not right, but the time gained from setting up a car is about right. These setups destroy your tyres in the race, you will find yourself sliding around and losing time, even spinning before the pitstop window if you don't drive careful.
This. 👍