Well there's no trick in speed. Practice practice practice will make you faster.

But here's one simple thing you can do, if your not already doing it.
It wont work in every case but when you get to a point in the scale where the upcoming note on the adjacent string is on the same fret as the one your holding right now, you bar both strings. That way you will only have to move the finger doing the pull off.

Ex. with the "most famous" minor pentatonic scale pattern : Every first note of every string is on the same fret. So you bar them all with your index and you do the pull offs with the others.