I bet non of you realy can play realy fast, reading what you write
Serriously, just strap on your bass and play, walk around and play, watching TV - Play, being on the computer - Play, speed will come after a while. PLAAAY!PLAY!PLAY.
If it sstarts to hurt tough.. stop playing. and either adjust your hand to a more comfortable position, or take a break, dont force it, let you hand grow up strength over time.
other than that:
PLAY!