[metanonsense]

I think the issue it the (literal) "head-of-line blocking". Some requests can be processed either "concurrently" like tapping a beer from a keg or in parallel by having multiple bartenders, but having all people standing strictly in line makes this difficult.

[trunc8]

Certainly in Irish pubs you wouldn't want the queue blocked while waiting a few minutes for a Guinness to "settle" before it can be served.