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Going to teach you the way I learned how to play really fast, easily:
play each note of the riff or solo slow and build up speed until you can play fast, without your picking hand and fretting hand going out of sync. Takes long ass time.
keep your picking hand playing at a regular pace, alternate picking, play 8 beats per note,in the riff. Your fretting hand with be moving at normal speed. When you're used to this, move up to 4 beats per note, then 2, then 1(where you'll be perfectly in sync with your picking).
It's easier this way because rather then having both hands trying to coordinate at once, you're just letting your picking hand do all the work and forcing your fretting hand to catch up, thus you learn how to play the riff faster much more quickly and effectively then you would the other method. This is also excellent if you want to play solos and such.
Hope this was helpful, and remember, speed is nothing if it doesn't sound clean. Keep practicing until it's perfect and concise, and most importantly, have fun!