With XSL, its easy to forget about the complex processes that go into parsing an XML tree. Some of these processes can be time consuming, especially if you cast a wide net over the XML tree by using the "//" delineation. This causes the parsed to examine every node in the tree, where many branches will have nothing to do with the node you are searching for.
I usually start wide to accelerate development, but then revisit later to optimize, as inefficiencies can creep up on you!