Hi Akash,
Most Test Centers are relatively small (meaning that they can only accommodate a specific number of Test-takers at any given time). That having been said, when you arrive, if there is an 'open' computer that is not booked for another Test-taker, then the Center Staff might allow you to begin your Test earlier than your specific appointment time (if that is something that you are interested in doing). In that same way, you might also be fine if you show up a little late (although I highly recommend giving yourself a bit of extra travel time on Test Day; arriving early is fine, but if you are running late, then the stress of having to 'rush' to the Test Center could throw off your Exam performance).
You can submit your Official Score to up to 5 Schools on Test Day for free - but you have to select the Schools at that time (there's an easy-to-use database available to you before your Exam begins, so it's neither difficult nor time-consuming to select the Schools). If you don't want to select Schools at that time, then that's fine, but you will end up paying an additional per-School fee later on to submit your Score when you apply.
There's a certain amount of administrative set-up before you enter the Computer Lab and before you begin the Exam (re: verifying your identity, putting your possessions in a secure locker, receiving your note-pad and marker, etc.). It's all fairly standard (remember, the Staff does this all day, every day as part of their jobs), but the amount of time needed will depend on the number of Test-takers who are there. Once you're sitting at your Computer, it shouldn't take more than a couple of additional minutes before you actually begin your first section of the GMAT.
GMAT assassins aren't born, they're made,
Rich
Contact Rich at: Rich.C@empowergmat.com