What is speed picking guitar?
You're picking on the string, not necessarily through it. This is a psychological reference point you can use to remind yourself to focus on reducing the amount that the pick travels away from the strings. This allows you to speed up the rate that the pick changes direction.
How fast is fast alternate picking?
Start at 60-90 BPM, and just remember: 'down-up, down-up'. Don't veer from these picking directions. Move onto 90-120 BPM. Now, the middle tempos are important.