If you have a solid race setup that works, you can try to improve on it by looking at the stopwatch. But at the moment there are only a couple of things you can generally adjust to be faster over one lap than racelength.
Tyre compound / pressure: Softest compound you can manage over one lap. You can try to fine tune the heating process by adjusting tyre pressures (or driving like mad before your timed lap). Lower pressures will give more grip and will heat up the tyres more quickly.
Camber: You can adjust the cambers purely on the cornering speeds (laptime), you don't have to worry about even tyre wear or even temperatures that much. Usually, a bit more camber than what's in your race setup.
Downforce: On some tracks, you might be able to accomplish a faster time by running higher downforce. In a race, high downforce might be a disadvantage because people running lower downforce can pass you on the straights. On the other hand, with low downforce you could get the higher speeds on straights and if you get lucky, get through the twisty bits unharmed. It all depends on the track that's used and the qualifying method (if you're allowed one quali lap you might want to aim for a safe result (higher downforce) instead of going off the track trying to push 110% (lower downforce).
Brakes: You can also try using higher braking forces and/or more rear biased brake balance during qualifying. If you can concentrate enough for one fast lap, there's some fractions of a second found here. Because over a race distance the overall time counts, brakes are setup a bit on the safe side.
If you have more than one try during qualifying, you can also try driving using more aggressive racing lines. Cutting the inside corners a bit more and going wider at the exit could result in a better laptime but can prove too hard to do over a race period.
Of course some time might be found from changing the suspension, transmission ratios etc. but they are very specific details should only be tampered with once there's no time to be found anywhere else
So overall, in qualifying you'll concentrate on making the best out of one lap, the car doesn't have to be that comfortable to drive, just get through that one single lap as quickly as possible. In a race, you'll want to make the most out of the race distance, with less emphasis on a single laptime.