Speed in Quant is a by-product of pattern recognition, not faster arithmetic.
Learn to see a question and know the method within 5 seconds. Percentages, ratios, averages, and time-speed-distance cover most of the section.
Keep a "silly mistake" tally. If more than 3 of your last 50 wrong answers were calculation slips, slow down by 10% — the marks you save exceed the time you lose.