What seemed like a few minutes to an accelerated individual went by much more quickly to an individual experiencing time conventionally. In fact, a person in normal time could not see a hyper-accelerated individual because he or she moved too fast. An individual in normal time heard a sound like a buzzing insect when in the presence of a hyper-accelerated person, which was really the sound of their speech, sped up to a very high rate.
When making the transition from normal time to accelerated, an individual's life span was shortened. In addition, they were susceptible to cellular damage, which could result in the person aging to death in mere minutes as a result of the slightest injury. (TOS: "Wink of an Eye")