See, in my head, “as soon as possible” reads simply as a more formal way of saying “I will respond at my earliest convenience.” Like, either way, this person is getting back to you as soon as they can, whatever that actually means.
We’ve certainly come a long way since the honeymoon days of You’ve Got Mail, the 1998 Meg Ryan romcom in which each new electronic missive set Tom Hanks’ heart fluttering (and vice versa). These days, in tech circles, you’ll hear tales of folk who’ve set their email servers up to automatically delete unread emails after a week – before going on holiday for a full fortnight. Others have reduced the OOO to a single word in the subject line: “Nope.”

If it’s not important and you’re just a little bit bored then you can amuse yourself with these fun facts until I return. When hippos are upset, their sweat turns red. Bananas are curved because they grow towards the sun. There are approximately 100,000 hairs on a human head. You can buy eel flavoured ice cream in Japan. A group of jellyfish is called a smack.
I can see why you’d have a negative reaction to it–that’s how I felt the first several times I heard about these kinds of emails–but I don’t think it’s actually rude (unless they’re saying “if I get emails from Ali G, I’m deleting them”). The wording of the email can be rude, but the general concept of this kind of email isn’t.