Explorer context menu is extensible (via COM), and lots of apps register themselves there for various minor features. The more you have registered, the slower it loads, because it has to instantiate all the components backing those menu items - and they're often registered as out-of-proc servers that require spawning.
Even if it shouldn’t, does it really matter? If you can’t change it, regardless the reasons, and getting an SSD is a fairly cheap/simple solution, then why wouldn’t you just get an SSD? Nobrainer.
