Bus service between parks can be very slow. It's just not a hugely popular thing to do. If I were at DHS, for instance, and wanted to get to the MK, I would hop on the CR bus. If I were at MK and wanted to get to AK, again, I would head to the CR and take their bus directly to AK.
At DHS and need to get to Epcot?? Either take the boat, or walk. Usually walking is faster...about 25 mins at a normal pace.
It is usually so much easier to head to a resort and then take their bus to the next park.
And yes, the buses to DTD are some of the absolute slowest. I have to say that when I rented a car, while at BW, and drove to DTD...I was astounded to find just how close it is. It seems to take forever to get to DTD from BW when taking the bus. But in a car, it takes about 10-14 mins!!!