Also shitty unity will cook your GPU even from rendering a square
It's not really unity that's the problem there, it's the devs not knowing how to work with it. Remember yandere sim? Watch this:
That describes more or less the average coding skill of the average hentai game dev as far as I have seen. (Even VAM, which is technically impressive in a lot of the things it 'can do' runs like shit because it's devs don't know BASIC SHIT like the guy in this video is covering, or maybe they do but just did not put in the effort to optimize it, devs like to make all kinds of dumb excuses for not optimizing their code these days)
The reason unreal and unity games tend to run like shit is because devs who make tutorials online tend to abuse the living shit out of per-frame-update code events (like event-tick in unreal or update() in unity), this means whatever code is there is run once every frame, do you see the problem here? the more code you put in there, the more code is run every frame, the more effort is put in by your pc to render each frame, each frame takes longer, therefore you get less frames per second) and these absolute noobs just don't know that doing that is a BAD IDEA so they just do it for like half the code in their game not knowing any better so it runs like shit.
If you want an example of what Unity can do in the hands of developers who aren't complete imbeciles, play Sons Of The Forest.
It's a good game, notably runs heaps better than The Forest did (because the devs were more experienced and didn't do as much stupid shit), looks incredible.
It's so easy to blame the engine for the faults of the developer, but the truth is, a game running like shit is
always the fault of the developer and not the engine. The developer chose that engine, he should have been informed about how well it can perform. The developer made the code, even if the engine has limitations,it is the developers job to work around those limitations to make their game play well in the first place.
So if this game runs like shit, it's probably because it was made by someone who didn't have the first idea how to code, wasn't particularly motivated to get good at coding, just did the bare minimum required to make their game actually work and didn't put 1% more work into it than that. Or maybe tried but just couldn't figure out how to fix their game and gave up.
Amateur devs like this is also why so many hentai games are just plain broken, a lot of these games, especially the 2d ones, are made by artists, not programmers. Or if they are made by programmers, they're made by incompetent ones who don't bother ever trying to optimize things or do things better than the very first way that occurs to them.
The truth is, we point at engines that are frequently used by novice developers, like unity, and maybe godot, and say "bad engine" because the games perform like shit, but the games perform like shit because of the novice developers and not the engine.