Although practice makes perfect, it is possible to over-practice. If speed is the prime motive then practising a lot is essential, however if you want to be able to improvise you must make sure that you don’t over practice. When you over practice, you develop “muscle memory” and when playing a fast solo the next note can tend to be the next note on scale rather than the best melodic choice. When you learn a scale, just learn it enough to know where all the notes are, then improvise over it using the scale to construct melodies. You may accidentally hit some wrong notes but this is fine - actually encouraged. It not only strengthens your knowledge of the scale plus the occasional wrong note can lead to new melodic discoveries. When I hear some players playing a fast solo sometimes I think “this guy knows his scales” but that’s all. Some people have facility but lack in emotion or melody, better to strive for “feel” and melodic content even when playing aggressively or at speed rather than impress with a show of pure technical bravura. There are other musicians who make up for their lack of technical ability by laying back and playing how they feel.
Keywords: guitar, learn, improvise, lead, solo, scale, notes