Whatever speed you play should be determined by the feel of the song you are soloing over and so a guitarist should be able to go as fast as hell or hang on to one note forever.
One of my favourite guitarists is Dimebag from Pantera. He can play lightning fast but often opts for some slow parts because they suit the song or sometimes just a bunch of atmospheric noise.
I like solos that have slow parts that build tension upto a fast part that is going somewhere. As long as it's going somewhere musically then it's just as good as a slow part. It's the people who go fast without any particular destination that suck.
A good example of Dime's great soloing is 'Cemetary Gates' which starts slow and then finishes with a fairly short blistering piece.