#1
i have a hard time finding which fingers are fastest to use so here is the tab and tell me what you would do i have been using just my first and third finger but i think there is a quicker way to do it

-15p12---------------12--------------------------------------------------------------------------
----------15p12h15-----15p12---------------12-----------------------------------------------
-------------------------------------14p12h14-----14p12-----------------------------------
---------------------------------------------------------------14p12h14------------------------
------------------------------------------------------------------------------------------------------
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#2
A good sort of "rule" to take to heart is to have one finger per fret, ie

12: index
13: middle
14: ring
15: pinky
#3
Use the index, pinky, and ring fingers. I know where I'd place them to get my speed, but I'll leave that for you to figure out. If mastered correctly, this should bring great speed to the table.
#4
If you're playing the 12th and 15th fret notes with your first and third finger, then play the 12th and 14th with your first and second finger. If you're using your first and pinky to play 12th and 15th fret notes then use your first and third finger for 12th and 14th fret notes.

I think playing that high up on the neck it's acceptable to use either fingering. If I was playing that same lick at say the 5th position(first finger 5th fret) I would use my first finger and pinky for the notes on the high E and B string , unless I was doing a bend , then I would use my third finger.