Fig 1: If a journey is split into two halves and the first half S1 travelled at the speed on the x-axis, then the y-axis gives the speed the second half needs to be cycled to get an average of 15mph. Given by this expression:
Going slow has a far bigger impact on average speed than going fast! Half the journey spent 3mph off at 12mph requires the other half 5mph faster at 20mph! The effect rapidly diverges as speed slows down.
As speed increases the energy required per mile increases with a square (or a cube per second) so to double speed requires 4x the energy. Thus the impact of the above means that total energy used in a journey rapidly increases if we spend time slower than the average we want see Fig 2. (Y axis has just relative units the numbers don’t mean anything.) So spending time at 10mph when we want an average of 15mph will result in us putting more energy in over the journey than were we to spend time at 25mph! 
The clear message is what I’ve heard from time trial cyclists that its “not the going fast but the not going slow.”
Now this is exactly what Aesop was saying. Short periods of spectacular performance are impressive but are highly wasteful and are easily out stripped by a slow but regular approach; the stream over centuries eroding the landscape versus the flood.
This applies to cars too. The car manufacturers would like us to think that cars with spectacular performance will achieve more but such performance will achieve very little if we spend time below the average we want. A Porsche driver wants to travel 100miles in an hour. If he spends 30minutes at 70mph he needs to spend the other 30minutes at 175mph! Or more sensibly we want to travel 60miles in an hour. The Porsche has to get out of an urban area and averages 40mph for 30mins of the journey, they will need to average 120 for the rest! While a car that simply found a route where they could cruise at 60mph the whole way would achieve the same.
The logic applies to any rate. To make money it is not making a few spectacular incomes, but the not ever making a little. Conversely to reduce rates and save money it is the little purchases, or the not buying things, that matters rather than the big buys. In chimps it applies to social status too: success has been shown to lie in saving energy by minimising skirmishes and putting it into big fights.
I think the main difference between working and middle classes lies in this too. The rich buy quality occasionally, while the poor make lots of cheap purchases. The rich work consistently at a reasonable rate, the poor accept low rates and hope to make up the difference by big wins in gambling/lotteries and occasional lucky deals. Capitalists know this and try to encourage us to spend a little often thus sapping our wealth.
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