Timing can be useful information indeed.
When a port (more generally, traffic) is blocked, it can be blocked by dropping (silent rejection) or rejected (an “unreachable” message (e.g. port unreachable) is sent back to the originator.
A silent drop causes when you see with 25/465 - a connection attempt is made and nothing happens until the timeout is reached.
A rejection causes a message such as “Connection refused” or “Port unreachable”.
That’s indicative of intended behaviour - you’re getting a connection and then the connection is being immediately closed.
You’ve varied the source, so something else you can try is varying the destination. Try the same command against e.g. smtp.gmail.com or smtp.office365.com. You should get an authentication failure, and that’s a strong indication it’s mailgun rejecting you specifically.