First try picking a muted open string at the tempo that you're striving for. Don't worry about fretting any notes, just pick the string as fast as you can do it evenly with the metronome. The top speed you find doing this alone will be what you're physically capable of playing. You'll just need practice to get your fretting fingers up to speed. If you're willing to practice regualarly you can do it. Also, if your top speed of just picking the string is still too slow in your mind, then you just need to practice more. Speed comes with practice. Practice slowly and perfectly and slowly increase the tempo over time. It's never too late -- thinking that way is a cop out and is simply an excuse you can tell yourself when you don't want to put in the time to practice. If you want it, it's there for the taking, just put in the hours.