I was listening to the WTC and noticed that many of the minor ones are played in a slow pensive tempo while the brisk ones are usually in major keys. I don't know whether that is a general trend or I'm seeing patterns that are not there but looking at the score of these fugues I didn't really see any reason why one would choose a slower or faster tempo. In fact, I tried changing the playback speed and many of them still sounded good at a wildly different tempo than conventionally played. So, my question is how do pianists decide on the tempo of a fugue and why is there seemingly a bias towards playing the minor ones slowly?