There is a STRONG tendency to rush things when trying to develop speed. And then things become self defeating. You MUST FIGHT the urge to rush. If you can play a scale at 120 bm 16ths, don't try to jump to 140bm and stumble through the same scale, you might be able to play it cleanly once or twice , but the key is consistency, you should be able to play the scale,lick , exercise etc... cleanly and consistently at any speed, then you'll know that you are on the right track. I think you'll find that the learning curve is steep initially but if you practice correctly, that curve will shorten before you know it. And remember to learn songs from "shredders" you admire. Once again, play the licks at your speed, and eventually you will reach the target tempo.