Most shops I know of these days assign a labor time to any given job. You get charged that amount whether the mechanic does it in half the time or takes five times as long.

Anymore, it's an internal benchmark for mechanics to build on the efficiency of their own work.

In my line of work, it may take me three hours to solve a client tax issue. I will bill for that accordingly.

If another client comes along the next day with the exact same issue, but this time I know the answer because I researched it yesterday, so I can solve it instantly, should the second client get charged nothing?

It does not, or at least should not work like this. If you can do same work, with same quality in less time than average, then pay rate is higher than average.

No.

It's literally right there in the sentence you wrote, thankfully.