I've been playing through my PS4 backlog lately and have noticed a very strange phenomenon, perhaps someone more versed in this area can explain. But I play a game like MGSV which to me has great visuals, runs at a full/locked 1080P resolution, 60 frames per second, is open world, has complex geometry and barely if ever dips below 60 frames per second. Doom 2016 also runs at a full 1080P/60 frames and looks insane. Then I get a game like Fallout 4 where it looks better than last generation but certainly not by much and that runs at 30 frames per second at best, with some major dips to the high-teens/low 20s. Deus Ex Mankind Divided also does not run too crash hot. The Last Guardian also runs at 30 frames per second but dips more than it should for the visual level is displays(in my opinion). Is it bad optimisation? Is it a poor choice of engine? Do physics/A.I(in The last guardian's case) play a big role in hindering frame rate?

Engine mostly, Fox Engine (MGSV) is a pretty new engine built for this gen, ID tech 6 is brand new ( Doom).

Fallout IV is built on a not so old engine, but well, Skyrim was the first to use it in 2011 and at the time it already rendered crappy graphics, so bethesda probably just sucks in that department.
The last guardian is built on a 10 year old engine, so well, the problem is obvious here.
As for the Dawn Engine, it's brand new, but sometimes things just go wrong with a new engine, ask Square Enix with the Crystal tools nightmare, maybe they'll update it to make it run better, maybe it will stay a nightmare, but not every engine is well built. Most engines aren't as great as ID tech 6 or Unreal 4.

This. I'd also like to add that the engine Skyrim uses is most likely largely based on the engine Bethesda has been using even before Skyrim. It dates back to at least Oblivion and possibly Morrowind. Bethesda tried to market it as a completely new engine but it's so similar to the one they used before (including the flaws and bugs) that it's not likely they created a completely new engine.

Budget and development time are probably factors as well, but engine is definitely one issue.

Yeah, it's the Creation Engine, which is based on Gamebryo, which was used for Morrowind and Oblivion. And a lot of other games over the years.

Its about engine and optimisations.

Bethesda makes Fallout4, and their reknown for makeing badly optimised & glitchy games (still good games).
Doom 2016 is made by id Software, a company known for great optimisation.

Its about how tallented the guys are, at optimisation that work at each company.
How willing they are to work to do so. How highly they prioritise it.

It's also about detail, animation and lighting used.





All the leaves rustle and branches respond individually to the wind, the same with Trico's feathers. It's all casting shadows too.
Fallout 4 has much more detail in the open world than MGS 5 plus you can change it on the fly, tons of items to shoot or displace and bases to build. A game like DOOM 4 doesn't have to account for that kind of change in the levels and can pre-bake a lot more.

To summarize the above posts and add one more bit.

Geometry is cheap in 3d processing terms especially when there's hardly any extra effects layered over the top. (MGSV)

Fallout 4 = ancient engine, badly optimized, lots of physics/cpu related stuff.

Doom 2016 = new engine made specifically for the game.

Deus Ex = lots of stuff going on related to cpu and physics.

Last Guardian = shed load of shader effects, not very frame rate centric type game.

Much of Fallout 4's problems come down to the fact that the engine does not handle shadows very well at all. It wasn't built for it, even though it has been heavily modified over the years. The biggest fps hits in the game that I remember are when you're in the city where the buildings cause zones of bright sunlight and deep shadow all around you, and spotlights. Spotlights used to be terrible if you had an older/weaker PC but they've actually much improved that by patching. Still, I always shot them out.

The other unique factor Fallout 4 has to deal with is the extreme amount of player interaction with objects in the world. The game has to remember and deal with all of that. I can't think of anything even remotely close. Skyrim is a drop in the bucket in comparison and it had so much going on that the PS3 couldn't deal with it late-game. It's really a game that shines best on a solid PC.

I suppose it's time for a new engine, though I'm kind of dreading that. Mod generation will be knocked back to square-one and development time will be extended greatly. I don't give a damn about the graphics but I think they ran into a wall with the engine in terms of doing stuff they wanted to accomplish.

Fallout is much more cpu intensive that those other games. Much more to keep track of as its an open world game having to calculate stuff not just appearing on screen but the some of the game world beyond it. You need to compare to similar open world games before judging it like GTA 5 etc.

I'm just playing Fallout 4 myself on xbone and crap there is a lot happening sometimes. Legendary Super mutants with their very powerful dogs and me early to the game with a lot of rubbish equipment. Plus those WTF mirelurks which look nothing like Fallout 3.