Obtaining OPT authorization takes around 90 days, whereas CPT is much shorter (~2 weeks). Why is obtaining OPT authorization much longer than for CPT?
OPT requires getting an EAD card, so that you can work for any employer. The application for an EAD, I-765, has long processing times, sometimes taking 5-6 months these days.
CPT, on the other hand, does not involve getting an EAD. When working on CPT, your I-20 authorizes you to work for a specific employer. Your school can issue the I-20 fairly quickly.