(Call of Duty) Split screen unplayable or 2 times worse FPS

I have an i5-14600K (14 threads, 20 cores, not overclocked), an RTX 4070 Ti (12 GB VRAM), 32 GB DDR5 6000 MHz RAM and a 2 TB Samsung 980 Pro M.2 SSD, the PC is just 2 months old.

I'm talking about split screen with only 2 players in all cases.

Call of Duty 3, 4: Modern Warfare, World at War, Modern Warfare 2, Black Ops, Modern Warfare 3, Black Ops II, Ghosts and Advanced Warfare all work flawlessly (aside from minor graphics issues in Black Ops) in the several hours of testing I did in each one despite only 3 and 4 being marked as "Playable" while the others are all only "Ingame" and they all run at stable 60 FPS and it's just perfect overall, looking (Resolution Scale at 200% since I have a 2560x1440 monitor, a day and night difference in graphics quality with no performance impact at all) and running even better than an actual PlayStation 3, really.

However, I have an issue with "just" one thing which is actually more or less the whole reason for me to use RPCS3 for these games which are also available on PC and that's console-exclusive split screen:

In World at War (2008, BLUS30192, 01.07), Black Ops (2010, NPUB30584, 01.13) and Black Ops II (2012, NPUB31054, 01.19), it's pretty much stable 60 FPS. I haven't measured it, but at least it feels that way, so that's more than enough for me because that's still as good or better than an actual PlayStation 3. 

In Call of Duty 4: Modern Warfare (2007, NPUB30588, 01.40), there are FPS drops in demanding areas of maps, I would say down to around 40 FPS. Not a problem, but it happens.

In Call of Duty: Modern Warfare 2 (2009, NPUB30585, 01.14), no problem in Multiplayer and in the first Spec Ops mission, but in the other ones, it's mostly around 30-40 FPS. When it's all clear in a place that isn't demanding at all, then it might be 50-60 FPS. It can ifteb even fall down to like 25 FPS sometimes, but it's still playable because a lot of console games run at 25-30 FPS to this day, as they always have, but it's not exactly good.

Call of Duty: Modern Warfare 3 (2010, NPUB30787, 01.24) is the worst for some reason. Multiplayer is fine, Spec Ops missions run like they do in Modern Warfare 2, but Survival Mode is completely unplayable because there's constant and major stuttering all the time as well as lower FPS, but FPS wasn't even so bad at all (probably around 45 FPS, if not even higher), it's the stuttering that made it unplayable.

Call of Duty: Ghosts (2013, NPUB31301, 01.16) and Advanced Warfare (2014, NPUB31553, 01.20) both have a similar stuttering issue in their respective modes similar to Survival mode from Modern Warfare 3, almost equally severe. In Multiplayer and Extinction in Ghosts, it seems perfectly fine, except that at the beginning of Point of Contact (Extinction), despite it being perfectly stable 60 FPS, it suddenly drops down to like 15 as soon as I look at the spot where the first drill is placed, then immediately back to 60. Really strange. In Multiplayer and Exo Zombies in Advanced Warfare, no problem except for the occasional negligible stutter, but whatever.

Now, I'm aware of how emulation works, why it's so demanding and why split screen is more demanding. You don't need to tell me that it doesn't mean too much that these games are already 10-15 years old, that I can't expect the same performance as I get when I run them natively and so on. However, this is still an i5-14600K, one of the latest and best CPUs out there. It's not like I'm saying this about my old PC with a completely outdated i5-4690K with just 4 cores and 8 threads compared to 14 and 20 on this one and so on. But, it's not even about the specs, it's just really weird and this is why:

For example, on the map in Modern Warfare 3 where I get extreme stuttering that makes it completely unplayable and considerably lower FPS in split screen, I get perfectly smooth 60 FPS with no stuttering of any kind when playing alone. Same with literally every other mode in every other game, couldn't be running any better at all. The mission in Modern Warfare 2 that dropped to 25 FPS despite both players facing the same way and being near each other runs normally at 60 FPS when playing alone.

In Modern Warfare 2, I tried looking entirely at the ground with the 2nd player when I had around 40 FPS and it instantly seemed like nearly 60 as soon as I did that. In Modern Warfare 3, not even that could make any difference and it didn't really help in the other problematic games either.
 
I know, when there's a second player, there's more to render on the screen and more things to process, so that has to be more demanding. I understand, but this much of a performance impact simply seems too extreme, all things considered. This kind of performance makes it feel as if there are 2 entire instances of the games running simultaneously than just a 2nd player. Split screen shouldn't cause literally halved FPS (or even worse) and severe stuttering, it's just too much and it shouldn't be that way. How come Black Ops (2010) and Black Ops II (2012) work flawlessly in split screen in huge Zombies maps which are significantly more complex than a basic, static Multiplayer map in Modern Warfare (2007) with just 2 players and no AI enemies or anything going on? Stable 60 FPS in the former, yet around 30-40 FPS in the latter somehow, despite release date and complexity. That just makes no sense at all. In both games (as well as all the others, regardless of modes or maps or anything), however, perfect performance when playing alone.

So, is there anything I can do to make performance a bit more acceptable in the games that are playable in split screen and is there anything I can do to make the unplayable ones (because of severe stuttering) playable?

Thanks in advance.