Um, no it’s not. If you think that roads are built with no consideration for anything other than if the infrastructure will support traffic at a certain speed then you’re just not thinking about things. Speed limits are set with many factors in that decision. Things like noise and safety are major parts of that. Someone else has already suggested a lame reason as a noise complaint made by a single person, but cars moving faster make more noise. Cars moving faster limits the time a car at a stop sign can safely navigate causing traffic at cross streets.

Regardless of what you think, speed limits are not set in place just to ruin your day because you can’t leave on time and constantly need to “make up time”. They are not arbitrary decisions just because you haven’t considered all of the factors involved.