The people interviewing you are busy. Even if they aren’t, you should assume they are. And the very first impression you can make -- even before they see you -- is to judge you based upon what time you arrive.
We shouldn’t have to explain why being late to an interview is a bad thing. Obviously, if you’re trying to impress somebody, making them wait at the outset is not the smartest idea. Even if you have a good excuse, the only thing some hiring managers will remember is you were the candidate who was late.
But what some people don’t realize is while showing up 5-10 minutes early is what you should aim for, getting there too far ahead of schedule can be viewed as equally disrespectful of someone’s time. Even if they leave you sitting in the lobby, now they feel rushed to finish up what they’re doing and tend to you. If you get there half an hour early, wait in the parking lot and go over your notes for a while.