I was writing an answer on this now-deleted question which basically asked 'Why do I get an answer quicker when I post on Stack Overflow than on Server Fault?'. This triggered me; was this just an N=1 case or is there really a pattern? Stack Overflow gets a lot more visitors, sure, but it also gets a lot more questions. That can't be the reason; the same applies to weekend vs. working day questions.
Quoting part of a comment by n8te:
I believe another factor is the nature of the questions. On Stack Overflow, if you post a quality question with the necessary MCVE, error messages you're receiving, expected output, etc, people who are experts at the language can often identify where you're going wrong almost immediately. With sites like serverfault, superuser, etc, the problem is more of a black box and often it's not immediately clear when reading the post what is the cause of the problem. Some answerers need to research it a bit and maybe find a source to link to.
Of course, on-topicness should be the sole criterion to determine on which site to ask a specific question. But it's an intriguing question nonetheless; do you really get quicker answers on Stack Overflow than on Server Fault or Super User? On which site will you get an answer the soonest after posting a question?