Guilty as charged.
I’ve used the (practically speaking useless) term “fast” to describe a bass neck multiple times. I’ve used it to describe my own Yamaha RBXJM2 John Myung signature basses, various Ibanez basses and a couple of others.
Based on the OPs question and many subsequent replies, the term “fast” is meaningless.
However, when I’ve used the term by comparison with other basses that did not have, in my experience, a “fast” neck, I think what I mean is that some necks, depending on their radius, shape (“c,” “u,” etc.), width of the fingerboard and finish (some finishes seem to slow my fretting hand a bit) seem to introduce resistance to some degree as my hand moves up and down the neck.
Case in point. For about 35 years I had a 1966 Fender Precision that had a fabulously worn neck and introduced no drag at all when I played it. I would jump on other P basses and would sometimes notice that the neck seemed to drag a bit as I played them compared to my 1966 bass.
I’ve tried to play Warwick basses but find that the neck is so round (to me) that I can’t move as fast up and down the fretboard (or reach the notes as quickly). It feels like it’s fighting me. Now I’m sure that’s due to my being accustomed to a less rounded neck and not an actual problem with Warwick basses, but that might suffice as a fast vs not so fast neck.
There are thousands of bassist playing Warwick’s who do not experience such issues (I know a few).
So it is ultimately meaningless in conversation to describe a bass as having a fast neck, but internally I know what I mean.