speed is useful for solos and riffs but has be used in accordance with playing skill and hand control otherwise notes get skipped and the song sounds cheaply made.
exerices that i use involve hammer ons and pull offs. start with ur hand in 9th position (first finger on 9th fret, 4th on 12th) on the high e string and play this.
-9h10-10h11-11h12-12p11-11p10-10p9-
once thru that then move to the B string and then work to the low E string then back to the high e. then move to 8th position and so on till you've gone over the whole fingerboard.
that should improve speed, and with distortion added you can hammer on the whole pentatonic scale without touching the strings with your other hand.
another good speed increaser is the song 'thunderstruck' by ACDC. sounds cool too