Once the bus leaves the resort, it's only about 15 minutes to the turnstyle area at MK. If you drive, you'll have to park in the TTC parking area and then take a monorail or ferry to the MK. The bus is usually faster to that park.
For Epcot and MGM, taking the car may save a few minutes, but the bus ride to those parks is only 10-13 minutes once leaving the resort.