There is value in being aware of the many reasons queries are fast or slow. What would begin making this useful would be a. A discussion about relative magnitude of the consequences, and b. where to look to determine whether this issue is affecting my query, and how much?

Question: what has been, in your experience, the longest amount of time it has taken to compile a query?

How much difference has it ever made, in your experience, to alter the default behavior of the query cache?

It would be an interesting exercise to turn off the cache, recompile every statement every time and make note of in which circumstances, if any, a perceptible difference was made.