Yeah, the loose nut behind the wheel is right!
The biggest lesson I learned was when my fairly well prepped '85 was down with a blown engine for the last race of the year. Perfect_circle allowed me to co-drive his car for the race, but it was nearly completely stock (I think he had exhaust). Original suspension, old tires, etc. What a difference from my own car!
Well, once I finished throwing a fit and trying to make the car do what it wasn't capable of doing, I settled down and started "driving what I've got". At the end of the day, I really doubt I could have turned in a better time driving my own car. That was when I learned that very little of your actual performance is related to what you are driving. Its very important to drive what you've got, rather than trying to make your car perform above it's capabilities... You can gain a lot of time by just slowing down and driving within the car's limitations...