An increased average speed will also get you a faster laptime. If your average speed increases from 89 to 90 mph, your laptime increases from 2:15.79 to 2:14.28.
There is no difference how fast you go or what lines you take because one lap at Brno is 3,3 miles. You can drive however you like but at the end of lap you will have 3,3 miles! Hope we all agree on that.
It's 3.3 miles if you follow the racing line, more (or sometimes less) if you don't.
So if you choose a longer line and drive with the same average speed, your lap time will increase.
If you can find a line that allows for a higher average speed, you can do more "miles per hour" or "distance per time" and win this event, even though this line is longer than the ideal racing line and not the quickest* way around the track.
*least amount of time