If you can play it slowly (and accurately), rote practice will speed you up if necessary. But, never ever play something faster than you can play it accurately. All you'll learn is mistakes.
There's one contradiction I'd advance to that, but only as a specifically-willful chunk of a savage metronome-based speed-building method I stole from Steve Morse. I know it works - REALLY-REALLY WORKS - and the students I inflicted it on who took it seriously caught some major zippies. I'll throw it up here -
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Every day when you're specifically doing "speed exercises" you have to find your daily baseline tempo, that at which you can play an exercise perfectly. This can vary a lot, 15% or more - tiredness, a cold, lack of practice, it's not pertinent for this right now. Each exercise has a different baseline depending on difficulty too. I like to choose longer licks for myself, 16 beats with some string-crossing that loops back on itself is perfect because it keeps me interested. Anything works here, Al DiMeola started with threes and fours then moved to combined ones. Regardless, choose one exercise at a time to concentrate on. What Morse says, specifically, is:
Play the exercise and alternate it with some scales or modes that you already know. Do this for five minutes at the baseline tempo, trying to play each note perfectly in time. Every five minutes, move up one bpm, and repeat what you just did.
After 30 minutes of this, you should have moved up 5 bpm from your baseline tempo. Remember what was the fastest tempo at which you could play all the notes perfectly. It may be your original baseline tempo, but usually you'll hit a higher number in a repetition like this. Now, take the fastest tempo and add 10 percent. Round off the increased number to the nearest setting your machine will display.
Play the exercise and alternate with scales at this increased tempo for five minutes, regardless of whether or not you are making mistakes. Turn off the metronome, and play the exercise one time perfectly, probably at a slower tempo. Now do whatever you want until tomorrow.
What this does is kick open the door to the possibility of always increasing-speed. Kinda rattles the nerves inside their sheaths.... I actually like to play them slow and fast, which ties in with Petrucci's "burst" method (which he doubtless got somewhere further back too). As my other biggest influence John McLaughlin says:
Speed and fluency are a combination of two things. First and foremost, in your imagination, you must hear yourself playing in this way, or it won't happen for you on the fretboard. Secondly, be willing to attack the problem of inarticulation through work and application of exercises.
Both Stevie & Mahavishnu Johnny are glossing over about a decade or so of 12-hour-day WORK, but they both really consider that to be the obvious & simpler aspect. This method is only asking 35 minutes a day, but you have to do it every day, all 365 of them, and it of course is in the context of at least a two hour-a-day routine. There are a couple of other specific "tricks" I find to be very useful pertaining to developing EQUAL NOTE DURATION - without which you can end up with three supersonic licks but you can't play frick-all else. We ALL know that kid.... :toothy11: But it's all hard, enough Mr. Bummer for now.