In Pumping Nylon, Scott Tennant outlines speed bursts as a way to increase speed and fluency on both right hand alternation and tremolo.
The idea behind a burst is that anyone can play fast, if only for a short period of time. Playing fast is like putting your technique into overdrive. But it’s also a feeling that has to be captured. Playing quickly feels like one motion, something that Philip Hii calls grouping: making many movements feel like one. Stanley Yates calls it getting the knack for something.
Playing guitar is not meant to be hard and frustrating. It’s meant to feel easy. Playing quickly for a short time is a way to capture that feeling of easiness.
If you’re having trouble with RH alternation speed or a fast scale. Try building it up. Start with just two notes (at concert tempo). Drill those two notes until it feels effortless. Then add one more, work until it feels easy. Then add another, etc. Be sure to relax completely between each burst. Start at the tempo the entire passage is meant to be. Don’t try to work the entire thing up by notching up the metronome. Just play parts of it at tempo from the start.