I have forever struggled when trying to improvise a metal solo over a backing track, like I would with blues or even rock.
According to various people, including my teacher, improvising a metal solo generally just doesn't happen unless one is very skilled already.
Is there a particular reasoning for this? Is it the speed of metal that tends to disincentivise improvising solos?
It’s not metal as much as it’s album-oriented rock, pop, and similar genres. “Album-oriented” is a kind of dated term related to music that is primarily meant to be listened to as a recording.
Fans of several related styles, including metal, enjoy music mostly as recordings and prefer to hear their favorite moments from the recordings reproduced in the live versions. They hear the same exact solo every time they listen to the recording and therefore mostly prefer to hear essentially the same solo live. This means the guitarists learn the solos note-for-note and essentially reproduce them live.
It’s not about the difficulty or timbre, it’s about audience expectations. Contrast with jazz and blues and jam bands where audiences expect improv and recordings of live shows are more popular than studio recordings.
In a mainstream pop or rock genre, you're trying to keep things relatively simple and predictable and catchy, so that it appeals to a general audience, and is easy to remember and sing along to.
In a genre like metal, you're trying to appeal to a niche audience who are used to the peculiarities of the genre. In the songwriting process you're going to try to be as intricate as you can with things like non-diatonic chord sequences, modal riffs and odd time signatures.
This means that improvising over the music becomes complicated, and you'd basically need the skills of a jazz musician to freely improvise over it. And then of course you run the risk of sounding like jazz or prog rock instead of metal. So it's both a difficult thing to do and a difficult thing to get right.
I would argue for two broad categories of melody for solos. Let's call one generative and the other composed.
Generative works form recombining small melodic units, motifs, figures, to realize the harmonic background. A lot of jazz solos work that way (scale and broken chord bit, enclosures, etc.) In Dixieland jazz I think of it a textural filling in. The process lends itself to improvisation. To a certain degree the results are lines that a interchangeable, or at least can be similar. Jazz fans may object to me saying that, but on the other hand there are those who will think it all just sounds the same.
Composed style in less a method of recombination and aims for a much more uniquely identifiable line. It requires more careful refinement of the line to obtain the unique quality. This type does not lend itself to improvisation, because of the careful editing and refinement needed for a good line.
The distinction between the two approaches is not about good versus bad. It's just a style difference.
I have forever struggled when trying to improvise a metal solo over a backing track, like I would with blues or even rock.
I think when you're working with blues or blues derived rock styles, you're working in a style, along with jazz, that has a generative approach as part of the definition of the style.
Metal, along with certain other rock styles (guitarists like Neil Giraldo or Elliott Easton come to mind, so roughly New Wave), favor a more composed approach. In that style the solo becomes an especially important to the identity of the song.
Your struggle is probably about the fundamental difference between the styles. I don't mean to say 'don't do it' or 'it can't be done.' I'm only trying to explain what may be the source of the struggle. If you're trying generative, bluesy style, it may simply be incompatible, style-wise, with metal. Perhaps the generative palette needs to come from elements other than blues licks and pentatonic boxes, etc.
