Conversation so far:
what does the zip in zip code stand for
“ZIP” in ZIP code is an acronym for Zone Improvement Plan.
Does ZIP mean anything else besides Zone Improvement Plan
The USPS intentionally chose the acronym ZIP to indicate that mail travels more quickly when senders mark the postal code on their packages and envelopes
What are the additional 4 numbers after the zip code for
The four additional digits after the zip code identify a geographic segment within the five digit delivery area, such as a city block, a group of apartments, among others.

Question: Do the numbers improve delivery speed
Self-contained rewrite: Do the plus four codes speed up mail delivery