[Question] How do guitar players improvise extremely fast licks?

For example some of those SRV licks on Texas Flood at El Mocambo. Is he just fiddling around with random notes from the minor pentatonic scale? Playing licks that he knows from prior practice and combining them? Or does he know what solo and notes he’s gonna play before the show? If it is pure improvisation, how does one get that fast and good? Obviously it’s SRV lol, but what can I do to get somewhere near that speed and level of improvisation? What’s a good practice routine to improve this?