Subnetting is the single most time-sensitive skill on CCNA, and most candidates practise it in the least useful way: slowly, on paper, without a clock. Replace that with three short drills. First, powers-of-two recall until block sizes are instant. Second, given a prefix, state the block size, network address and broadcast address in under fifteen seconds. Third, given a host requirement, choose the smallest prefix that satisfies it. Ten minutes a day on these three drills beats an hour of unfocused practice. Once you can hold the pace, move to full-length timed practice tests, where the real risk is spending four minutes on one addressing question and running out of time on the configuration items at the end.