After you are comfortable with the concepts involved, time yourself while practicing questions. Every session, try to shave a few seconds off each question. For instance, maybe start with two minutes per question. Next time, reduce this by 10 seconds. Next, knock it down to 1:45, and so forth. If you don't answer the question in time, mark it as incorrect whenever you finish and redo it later. Use the GMAT timer
, since you will have to sit in a chair and use a computer desk to incorporate it in your question practice. This is good practice in itself, as you will take the real GMAT in similar conditions.
As Hussain15 stated, you will improve over time as you continue to practice with the OG with a timer and an Error Log