My metaphor is a diner. The Resource Diner.

via Adam Machanic : How Queries Are Processed (A Month of Activity Monitoring, Part 13 of 30).

I love this visualisation of how delays can happen during the processing of a query.

It reminds me of another metaphor. Earlier in order to have more performance in a car, you would simply add more cylinders to the engine (add more CPUs) or increase the size of each cylinder (increase clock speed) or pump in more fuel add memory.) Nowadays you have fuel injection systems adding the fuel under higher pressure and in more than one squirt, and you have multiple turbo-/superchargers, and modulated ignition sparks, and a whole host of other things. So, you really do have to be a rocket scientist in order to know which dial to turn in which direction and how far in order to improve performance in each and every scenario.