I have found that my speed has improved dramatically over the last few months. I only just noticed it the other day when I was adding some fast, emotive fills into my solo's. Rather than using the usual method of hammer-ons and pull-offs to go really fast, I picked every note instead. Even though I am technically playing slower, I am being much more accurate. This equates to a sound that actually sounds faster because you are playing precise 8th, or 16th, notes rather than a mish-mash of differently timed, chaotic notes.
Don't get me wrong though, hammer-on and pull-off stuff can be very precise.. it's just that I tended to use them as a crutch when I wanted to shred up through a scale. Using a combination of the two shredding techniques is where the real speed lies, in my opinion. But everything must be ultra precise and accurate. Then you can throw in a few slow, out of time, sloppy notes here and there and they will sound awesome.. lol.. I do it a lot now.
I find that I hold my pick differently when I know I have to shred. I hold it between only my forefinger and thumb, as compared with holding it with forefinger, index finger and thumb for playing Rhythm guitar. A little while ago I got an infection in my right hand pinky finger. This meant that I couldn't anchor my hand to anything.. and you know what?.. it actually helped my picking. I also tend to hold the pick a lot more loosely than I used to before.. There's no need to hit the strings hard when you have so much amplification and effects processing these days.
If you don't like the idea of a Metronome.. find some sort of drum-loop thingy.. there's probably something around on the net somewhere, or you may already have something that can do it.. My Zoom GX1 does it.. anyway.. what I do is a pick a particular groove and tempo and let the drums play. Then I play basic scales over it, in time with the beat. Start with something around 60 bpm in 4/4 time.. it's slow but the beauty of music is that you don't actually need to play only those 4 notes/beats per bar. You can play less or more than that.. For example, you could play 8 half-notes, or 16 quarter-notes, or 32 8th-notes etc etc.. Because the tempo is reasonably slow, hitting 8th and 16th notes is actually possible for many more people.. and when you base everything else around the common (4/4) time, playing 8th-notes here and there sounds awesome.
Speed isn't everything. Precision and Efficiency are more important.