It’s about accumulation, not queue efficiency.
Fastpass works because it makes the individual items in the queue more efficient in “consuming” something. It’s not so much a queue like a processor waiting for instructions to execute as it is people waiting in line for food.
In the case of people at Disneyland, it allows them to maximize their fun.
Think about a processor accepting instructions. Each instruction is waiting to get executed in the queue, to perform its task. Now change it up – imagine each instruction is waiting in line not to execute an instruction, but to get something from the processor – each time it hits a processor it is rewarded with a gold star, and its job is to accumulate as many of these as possible.
Fastpass is like allowing the instruction to go somewhere else, to a different processor, to get a gold star there, before returning back to the main processor to get the golds star from it.
In the case of users at Disneyland, they’re interested in having fun – accumulating ride experiences. The Fastpass allows for a maximization by allowing the user to find a different ride with a shorter line, so they can accumulate more in a shorter time.