One simple and low in resources method will be to use pre-blurred sprites in relation to their speed. See the following example:
An other thing that comes to mind is to somehow link the horizontal velocity to the effect Horizontal Blur and the vertical velocity to the Vertical Blur. But this is dependent with the method that you use to move the bullet (bullet behavior, physics impulse, every tick move x self.X+(something)*dt)... In every case you'll have to figure a solid way to do the linking and it will surely be more taxing to the cpu, calculating the effect for every sprite and all...
But maybe there is a simpler and more efficient way out there that I don't know of