If I receive an email and it’s something I can quickly answer on my phone while waiting for the bus etc., I’ll do so and you’ll get a quick answer. If the email requires me to sit down and compose a long response (or worse, read a paper, or find and run some code) the email gets put on a priority queue to deal with during dedicated email-answering time.
If I receive an email with multiple questions, and one of them I can answer quickly, I might fire off a partial answer (under the theory that a partial answer now is preferable to a complete answer much later).