We usually stay at OKW, which reputedly has the best bus service of any
DVC resort...but we always drive to the parks. We not only drive, we usually drive and walk, rather than driving and tramming.
One reason we drive is our normal park visitation scheme, which is to go early in the morning through lunch, then back to OKW for nap/pool, and then out to dinner later. That means we arrive at just about any park, including MK, early enough to park within easy walking distance. In fact, normally we can walk faster than we could tram. For us anywhere but MK, from the time we reach the bus loading area, we are home in 10-12 minutes. At MK, that's a little longer because we have to monorail to TTS and then walk, but it's still quicker than the busses. I'd say we save 30 minutes on each round trip -- and we typically do two round trips per day.
This last trip, we experimented with driving to Epcot and then monorailing over to MK. I don't know if we saved any time that way or not (I doubt it), but it was certainly more effort than it was worth.
It is NOT all about speed, certainly. There are times when we take the boat from MK to TTS, and we love the boat ride from OKW to DTD. We almost never drive to DTD unless we're on our way home or the weather is iffy, or if we're going to Cirque.