Depends where in the park you are. You know...in all my trips to WDW, I have never taken the bus between Epcot and DHS. Will have to give that a shot in August...purely in the interest of getting information.
But..if you're in World Showcase, then it may be much faster to take the boat, or walk. The boat ride, and only the boat ride itself, is about 25 mins long. But, you could very well have to add in some wait time if the boat isn't right there, boarding.
I will say that my friend and I were leaving the International Gateway and heading to DHS. We figured that since the boat was right there, we would hop on. Not so fast bucko!! They had just shut the doors...they very comically told us to wait, another boat would be along in 15 or so mins. No, we yelled...we were walking and we would beat them to DHS. Yeah, right they responded..no way. Well, we all left at the same time. They did beat us by about 90 seconds to Boardwalk. But the last we saw of them was as we headed through the Village Green and they were on the way to YC/BC. We walked along the canal, on the sidewalk, at a nice normal pace. The next time we saw them, they were rounding the bend and we were on our way to bag check at DHS!!!
So....walking is faster, unless you walk very slowly.