If you have overall good performance in a game but it dips over enough circumstances that come up, for the sake of a consistent experience you can lock it down to a framerate that is below what it is actually capable of a lot of the time. When you do this, the game is usually much more responsive than a game that struggles to hold 30fps most of the time.
Horizon 2 has the feel of a game that was locked down from something higher, but I can see how in some areas and weather effects and higher number of cars, it could easily have dipped below whatever it is capable of when driving in an open field. Still, the performance seems unwavering despite a wide range of activity, which makes it feel "artificially" locked down to 30 for a uniform experience.
In the mainline series, there is a lot smaller range of variables for what is displayed between different moments. They control how many polygons are seen for every single turn, they control the number of cars, and they can check how both plus added effects change things across every inch of track. You can't really exhaustively do that in an open world game, so the potential for optimization is far higher with Forza 6.
It is me or you are mixing three different devs in this post.
Just to be clear:
Halo 5: i343
Forza 6: Turn 10
Horizon 2: Playground Games
Yes, I understand, but we have to draw our ideas of what is possible on Xbone with Microsoft-supported ground-up development from
somewhere. My comments on Forza are added perspective that I don't think major sacrifice is just some hard law we'll always have to expect and live with on the console. Developers can achieve nice things, and there are some present and anticipated examples, so not all hope is lost for Halo 5.