I like this site, but what makes me despise sites are things like this:
Oops! Your answer couldn't be submitted because:
new users can only post answers every 3 minutes; try again later.
Please consider disabling such a useless feature... sorry for being a rodent about this but I find such messages Annoying!
There was a specific incident that I recall (possibly one of many) where a new user created an OpenID using a junk e-mail account, and proceeded to create dozens (maybe hundreds) of junk spam questions, filling up the front page. At the time, the moderators were completely swamped trying to kill them all, and firstname.lastname@example.org was taking too long (IIRC it was after their "regular" hours) to ban the account.
This measure was put into place so that troublemakers can be more easily managed by community moderators until the site admins can take action.
It's because the system can't be sure you're not a "spammer". That's not to say you are one, it just can't be sure. Now, I still get "Comments may only be submitted every 30 seconds" or "comment votes may only be submitted every 5 seconds" fairly often, and i have a couple thousand rep. You wouldn't want spammers sending 2 dozen questions out at once or adding 5 answers that make no sense in short order do you?
In short it's so that moderators can minimize the damage of spammers if they should appear.
fwiw - once your rep hits a certain level, this restriction goes away.
I find the amount of time it takes me to answer on any of the sites entirely dependent upon how much I have to look-up vs what I know right-off. Sometimes I can answer in 40 seconds, other times it's 40 minutes.
That rule is there to stop spam - think about it rationally: some spammer uses cheap labour or a fake OpenID provider to create gazillions of users on the system. He starts using them to post spam answers.
Having a time limit like this keeps the spam output at a manageable rate and gives the mods time to clean up the mess and ban the new users or block IPs.