Do we have an explanation on why denuvo lowers the framerate on games?
Lemme try an analogy. Imagine a game engine as a factory in the strange land of PC. It's highly tuned for efficiency, and while it's a big building, it's laid out logically. And it's 100% automated - no human employees except the manager who comes in once every month. Early in the morning, all the previous day's packages get shipped out. In that time, some of the machines can start, but full production can only commence once the warehouse is emptied out. Once production has started, things can only go into the warehouse, not come out.
Now, you want to rob that factory. You get out your x-ray camera (a part of every master thief's toolkit in PC land) and take some photos of the place. The first thing you see is that the only lock in the entire building is a Steam™ lock on the front door, and someone posted a guide to make universal keys for those 10 years ago. So you literally just walk in there with your fake key, go to the storage room and take whatever you want.
What can a factory owner do about this? Sure, they could get better locks, but thanks to the x-ray vision, that's only a minor hurdle. They get one more day of selling their products undisturbed while you study the locks, then you have fabricated keys for those too.
In comes Denuvo. They transform the entire building into a maze. Go through the wrong path and the door behind you closes and you gotta head for the emergency exit and start over. They replace production lines with bizarre Rube Goldberg machines that produce the same results but also have ways of detecting anyone in the building with a fake key and forcing them out. This is already pretty annoying, but the path is still pretty clear because 90% of the maze's corridors are empty, so in every empty corridor they put a machine that looks like all the others but doesn't actually produce anything. Those are never turned on, so they don't waste any energy, but you gotta go in and peek around the corners to see that.
Of course, if they did this with
every production line, the factory would spend an eternity getting a single product out while wasting copious amounts of energy. So a while before the plans for the facility are finished, the owner sends Denuvo a draft and one of their engineers goes over it and finds the ones that don't matter. Of the tens of thousands of machines in any given factory in PC land, there's hundreds that need to run for half a minute at the start of the day. Denuvo picks out some of those and replaces them. Their new machines take 10 minutes for the same job, but that's perfectly acceptable. During the rest of the day (when you can't get in the warehouse anymore), none of Denuvo's machines are even on. If they all finish before the shipping process mentioned earlier, the slowdown doesn't hold anything up at all.
Usually, this works as it should. But with such a complex system to go through, there's always the potential that Denuvo's engineers make a wrong decision. Or maybe they make the right decision at the time, but your plans changed, and one of the machines they picked now holds up every single 1-hour production run. Then, replacing exactly the same machine with exactly the same replacement that would've made no difference worth mentioning with the original plans, they'd be making every single product take 15% longer to roll off the line.
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If Denuvo makes a game slower, that's how it would happen. Not because (as many people seem to think) the technology needs to constantly run in the background eating a large chunk of your CPU time, but because an error causes it to waste time when you really can't spare any. Developers could deal with that by performance testing the final protected build and yelling at Denuvo if they see it's no good. But they'd have to have that build ready early enough that they can afford to wait for the response.
The reason I keep saying "if" is that as far as I'm aware there's still no concrete proof that's ever actually happened; you'd first need a performance test with proof that the only difference between the game versions being compared is that one has Denuvo (cracked or not) and the other is DRM-free. In DMC5's case Denuvo is a likely explanation but not the only one.