Probably maintenance, or in many cases, irrigation capacity and occasionally water quality. A real world example - I was building a hole and knew it needed some cross slope to fit the land.
Around that time, I happened to be at a local country club, digital level in hand and saw where their fairways held shots at 8% on the same Bermuda grass we were using. So, I designed that fw at max 8% cross slope. In a few areas, the shapers took a few liberties, and it exceeded 8% in spots.
However, they have run into irrigation supply issues this year on the new course, and the fw is very firm, and the 8% slope kicks balls left, and into the bordering creek, despite a small "save" lip, which apparently some balls bounce right over.
So, did the design support the unintentionally firmer conditions? Or, should I have been more conservative to anticipate that?
I recall Jay Morrish telling me that max cross slope should be 5% in the LZ. Hurdzan wrote that it should be 4% slice side, 6% hook side. JN wrote it had to be in the "single digits" presumably a max on 9%. I was looking to avoid boring template design, and wanted more contour, but if we want to consider firm and fast and/or future water shortages, it looks like I should have been more conservative.
So, that is a real world design issue related to firm and fast. Perhaps the phrase ought to be, "firm, fast, and flatter."