Enter the real of the motormouth. A fast paced voiced is all about words-per-minute, a constant barrage of information that is just on the right side of intelligible. If you need a script read fast, this is the way.
There are times that call for speed. Or maybe you've got a character who has such an over-caffeinated cadence that they could make Dave Grohl or Quentin Tarantino blush. The fast paced voice is all about getting information out there quickly while the rest are just able to keep up. If you character or script calls for something to be read as if they're permanently in a hurry, this is the voice type you need.
Some accents just have a natural cadence to them that puts them in this category.(Scottish and Irish accents, for one); others are more individual examples of crazy-fast talkers. Regardless, some of your projects may call for someone who sounds like they've been put on fast-forward. Crazy scientist types are usually a well-established trope that exemplifies this voice type, as they tend to speak in barely-understandable jargon while the heads of the main characters spin. Sometimes, you may put out an ad with a character that's so over-the-top excited that they just can't refrain to talk fast (and you make the most of a really short spot time).
These voices tend to be slightly on the higher-pitch end of the scale. While they may have characteristics that overlap with other voice types, their main feature is always speed and intelligibility. Even as it goes supernova, the fast paced voice is always clear and understandable.