How hard are you plucking the strings? Although I'm not the fastest of players either, I have found that using a [i]lighter[/i] right hand touch helps me to play with more speed and fluidity. Playing hard (a punk trait if ever there was one) can make you tense up, and thus slow you down.
If there's a piece I don't need to nail for a gig, but would still like to conquer anyway, the dots go into my 'Bass Challenge' folder (and I maintain a corresponding iTunes playlist). In my practice time I will sometimes pick out a song from the folder and have another go at it. If I crack it it goes in my general alphabetical song folders (yes I am that organised), otherwise it stays in there for another day. A few pieces have made the transition. The latest song to get the treatment is the Jacksons' 'I Want You Back'. It's still in the 'Bass Challenge' folder, but not for much longer I reckon.