What does it mean if a professor does not answer your email in time?
I wrote him an email to ask quick but important questions, like, when do you want me to start. I didn't get an answer even after a week. And similar things happened before as well.


Some professors are notoriously bad in answering emails. This does not necessarily mean anything. Don't fret about it.


  So I want to ask that as a professor, sometimes will you (on purpose) to NOT answer emails in time?


I'm not a professor, but, no, this is not how I would expect an adult person (in a management position, none the less) to behave. If the professor is indeed having second thoughts about you and decided to just not answer anymore, I would say you dodged a bullet there.

However, I really think that this is unlikely. Just send a polite reminder, or propose to have a quick chat e.g., over Skype, at a time of the professor's convenience.