The only way to play fast, is to play slow.
General left hand speed excercises I'd say, would be just running scales and arpeggios and stuff, simple stuff.
but, when it comes to playing a song, it's a whole different ballpark, the songs phrasings and patterns might be totally different than any arpeggio or scale that you are used to, and thus your fingers need to work more to get the songs movements down, and the only way to do that is to practice *VERY* slowly, lock it in your muscle memory, and then you should be able to play it as fast as your fingers will possibly allow.
of course, you don't want to rely too much on muscle memory and stuff, because then you stand to risk getting caught up in the patterns and movements and then you need to start from the beginning and stuff.
I'm not making much sense I'm sure, but think of it like this, a lot of people don't know the alphabet, they know a song, you ask them, what letter comes after P chances are they will think for a moment "A-B-C-D-E-F-G" singing the song to themselves, until they get to P in which case they will respond Q of course, but they had to go through that song to get there, it's a similar thing if you only internalize the movements, and don't actually learn and break down a song, and really learn the notes and their relationships to eachother...etc etc blah blah blah.
hope that helps
