I have some experience with really, really, realy, fast fuses! They were to protect transistors in a very expensive and hard to repair motor drive circuit in a missile guidance system. Turns out, there are fuses that are fast enough to protect transistors, but they are themselves transistors, and they cost more than what they protect. It was worthwhile in this case to use the fuses because the cost to repair a blown circuit, reassemble and retest the guidance system would have run to tens of thousands of dollars. Better to replace the thousand dollar fuse.