The first thing that jumps out at me is your use of the words "concepts" and "basics". While yes, to score high on this test you need to understand the concepts and basics that are the building blocks of the test, the GMAT is a reasoning game and so to rock the test you need to be good at playing the game.
In other words, to get faster, you need to get better at playing the game. In quant this can mean learning to find quicker, more efficient, or tricky ways of finding answers. In verbal this could mean such things as getting better at eliminating wrong answers, learning what to note carefully and what to breeze through when reading passages, and getting better at finding decision points in SC questions.
To speed up in quant, one thing you can do it look at your practice tests and see which question types are taking you longest, whether you get them right or wrong. Then practice those types. Many people use error logs to track how they are doing and figure out what they need to work on. By getting quicker at handling those types you will save yourself time to do more questions of other types.
One way to work on specific types of quant questions is to use question banks that can be broken down into categories. One such question bank is the BellCurves question bank, which can be accessed by going to the BellCurves site, going to the GMAT section and setting up a practice account. That's a great source of tricky quant questions, thousands of them, and you can work category by category watching your time, your accuracy and your understanding.
The explanations on the BellCurves questions are not always the greatest, but anyway, you can work on developing your own approaches to handling the tricky questions. A great source of good approaches is this forum. The experts often offer multiple approaches to handling a question and you can learn a lot from seeing how they do things.
So doing these things could be a way for you to take your quant answering skills to another level and speed up your performance.
There are other question banks that can be used for verbal practice. One is the Veritas bank, with which you are already familiar. Once again figure out what's taking you the longest, and when your ability to concentrate is the most challenged and focus on those things.
One thing to realize is that really this test is less than four hours long, which is a little long, but not that long. Yes, after a few hours continuing to concentrate can be a little challenging, but maybe if you remind yourself that you only have an hour to go, you will be better able to continue to focus and get right answers. One thing people do, I don't know if you are one of them, is get caught up in worrying about the fact that they are losing focus. I say stop worrying about that and be concerned with finding something to do to get a right answer. Another things people do is get angry, panicky or freaked out. None of that is going to get you right answers or help one bit. What will help is to get involved in a question somehow. By doing that you will tend to naturally start concentrating again.
This answer is not the end all for your GMAT success. Maybe some other people will jump in with some more ideas for you. In any case, you could do these things and continue to assess your performance, and you can continue to get advice as you go along.
