All of my pieces have now been tagged as either fast or slow. Yes, there are a number of pieces which fall in between, but basically, if you are looking for music like "Incantation", then check the fast category, and if you want something like "Isolation" then check the slow category.
You can use the search tool (little magnifying glass in the top right corner) to combine the words fast or slow with album names. For example, if you like the "Transitions" album, and only want to see slow pieces from that album, then type Transitions slow and you will see the results. It is not necessary to add a comma. Additionally, if you wanted to see fast Christmas music, then just type Christmas Fast.
Other tags will soon be added to allow you to refine the search further. I will definitely add whether the pieces is in a major or minor key. However I am not sure about adding key signatures or time signatures. Do you think that is something you would use? Would you search for music using that criteria? Let me know in the comments.