Also, there's a pecking order. If you are some unknown founder who really needs Conway you should respond as fast as humanly possible. On the other hand, if Conway wants something from Steve Jobs he probably should respond quickly.
Which is not really what that point was saying.
I just think that the time between sending and email and a reply is not a good measure of how important you are to that person. Certainly, if someone replied within a few seconds of my email to them I'd have to pause and question why they responded so fast. Are they literally desperate for my input? Why? (of course, usually being desperate for that input is fine, and part of being excited to have a chance with someone who can really help you - but occasionally it is a marker of someone dodgy).
Basically: it seems something of a shallow way to judge things.
wait half an hour, and the emailing session of your potential investor could be over, and your response would be buried under other important bullshit in their inbox.
I've done this in the past and been the recipient. It just goes bad.
Busy people work on short terse communication, unless they are actively in a conversation with you. And, in general, they will appreciate the first few emails you send them being of the same - condensed and full of content.
Never worry about being rude by not saying please every other word.
On another note, I mildly dislike this one:
Show your target respect by responding to everything immediately. Just because the VC you're emailing might not get back to you immediately, doesn't mean that you have the same privilege. Ron Conway famously makes immediately email responses a pre-condition for investment.
I'm a busy guy, I get a LOT of work email each day. And yet the people we work for seem to see slow response as an indication of laziness :S Even though it takes them a week to respond. Yes, you're important to me, you are potentially worth a lot of money to me and I really want to keep you happy. But give me a fracking chance!
So, investors, cut founders a bit of slack :)