I don't think it matters much what you do as long as it challenges you and you keep at it. Having some background music really helps cause it takes your attention off every little mistake you make and gets you used to playing in time.
One thing I found that made a huge improvement in my speed was learning scales with 3 notes per string. Previous to that I played for about 10 years and couldn't get any speed at all. Then I started learning all the scales backwards and forwards, partial scales, string skipping etc. and within 2 years I doubled my speed and suddenly had a better understanding of the fretboard. Like, knowing where the notes are that you want to hit and not fumbling around and stuff.
You've just gotta keep at it. I've been playing for about 30 years now and some of the stuff I tried to learn when I was in my teens like Kiss, Sweet, Deep Purple etc.. sounded so impossibly fast. Now I put the records on (yeah records :) and they seem like they're in slow motion. Even Van Halen who I thought was the fastest player alive after I heard his first album, sounds kind of slow to me now.