Many guitarist do not know or have a good strategy for building speed on the guitar. In fact, many guitarists try to build speed using an approach that is almost the opposite of what they should.
There is a lot that goes into building speed, however, there is one element that is more important that anything else.
Accuracy.
Without accuracy, it doesn’t matter how “fast” you can rip through a scale or arpeggio. If it doesn’t sound clean (which is a result of accuracy), it will fall flat.
Practice for accuracy before you practice for speed. Accuracy will actually build speed faster than just trying to play faster (which is what most guitarist do).
There’s a lot more that goes into playing fast and building speed, but this is the way to start.