I think most of the time, slot game providers' remuneration depends on the job they have done with the casino operator. If it is a house game, most likely, casino will pay the developers/providers a one time payment.
On the other hand, I think providers like Pragmatic, NetEnt, YGGDRASIL, etc. are bound to get percentage based from the revenue that the slot has made.
I don't think casino will offer fixed rates (per time unit) for the provider, just imagine if the game does not became popular to their players and close to no one is playing it. So, I think percentage basis is what casino thinks are the best for themselves.