I wonder if the amount of time we all need to zero in on a usable build/car will shrink as we get more familiar with the cars/parts/game?
Personally the first challenges sucked up TONS of time, because I was building tons of cars and playing with parts to create a fast car. I knew the muscle cars, but I had never tried to build the fastest car possible, just a fun, fast car.
The most recent one took a long time for me because (as I think I said elsewhere earlier) I didn't know Suzuka well. I had been meaning to do so for a long time, but this challenge came along first, and I cut my teeth learning the track with extremely difficult-to-drive cars like the Charger. (Or at least difficult to drive CONSISTENTLY.)
Finally I spent a lot of time tuning speedway cars, because I have very little experience doing so.
Right now I have three B700 Z/28's, two Cudas, two Chargers, two Shelbys, and one of each of the others. Given the variety of tracks we've run so far, I imagine I'll have at least one car that will run very well on any given track from here on out.
Of course, a lot of you guys spend a ton more time tweaking than me, so maybe the workload will never go down for some of us.