Not really, there is no speed penalty. Also this is not an extension this is a Firefox tweak/hack. If you look at the instructions your modifying the Firefox config, and this does help with the memory leak that some extensions cause.
lol, unfortunately with google talking to the leading music industry executives, there will probably be some kind of DRM, and some kind of over-priced subscription service. I think that with this new announcement we are seeing a turning point in the Google saga. With this new turning point I believe we are loosing our lovable Google in favor of a Google that wants to just make money.