What is meant by a "fast" pre amp

To me fast is the equipments ability to track the musical signal.
Usually if the equipment sounds fast it's not because if it is fast it will sound like misic.

Brf, Is the technical definition what you wanted? The last thing you want to do is buy equipment based on specs or some long elaborate explanation. The tweak industry is full of this kind of talk with space age materials and methods that only a NASA engineer would know.
The real question is; Does it sound like music?
Buying on specs only leads to disappointment.

All of this can be summed up in one word, MUSIC.

A preamp could have all of the best specs in the world, but if it doesn't sound like music, you have nothing.

If you think your equipment sounds fast then it probably isn't.