True, though that's a very perverse incentive, as you're suggesting PornHub's best bet is to waste as much of their users' time as possible in getting to the right video, without frustrating them enough that they leave the site and go elsewhere. I suppose they'd get more search-page-result ad displays in, but arguably it makes them a worse porn site. The fact that they're the by-far most popular suggests that they don't provide a worse experience than other sites.

(I know PornHub releases a lot of statistics annually, I am sure average time for a user to... achieve their goal on the site for the day... is a figure they have released before.)