it is using the same old engine classic wow was using. you clearly dont know how game engines work. They keep adding to them, call of duty was a great example till the last one that uses an entirely new engine.
Ever since the pre patch for BFA performance went down the drain. During legion i could max out the game with a 7700k and a gtx 1080 and very rarely drop below 144 fps. But now my 9900k and 2080 Ti im lucky to keep 80 fps in cities with lots of players like boralus or stormwind.
Your ui is poop. I got i5 6600k, 16gb slow ram, cheap ssd, low end mobo and rx 580 atm because my 480 died few months ago. I got 0 problems with fps, i even caped max fps because GPU was too loud. Go watch few youtube videos on how to setup your game settings. Also reconsider addons choice, as i wonder how can you see anything in this mess.
There are 2 options. Lower your details to gain fps or stay maxed and feel the pain.
Classic client is based on BfA one, while started as a Legion client fork. And to make Classic from vanilla data they had to convert everything to the modern formats.
Because they increased the complexity of systems and their data design enforces integrity over else so the more actors the more it becomes a bottleneck. This isnât a client issue local hardware can change. It up to their backed design to remove this absurd overhead caused by overgrown complexity. They even comnented on it in one of recent interviews.
Here is my question that I would like answered. Are you confusing Latency with FPS?
I just recently bought a new computer with 2080 RTX (not TI) Iâve got a i9-10900k and 32GB ram installed. I have wow at max setting with no issues, not even 40v40 bgs are a problem FPS-wise.
Have you maxed out how much FPS you can have in game and in the background? WoW has that already set as an option for you as a standard.
And in some cases depending on your CPU, multi-threading can be the bad guy.
EDIT: You should also consider using more Weakauras instead of the UI youâre using right now, you have way too much stuff going on your application. Consider using ElvUI and follow some guides on how to set that up proberly and then add whatever weakaura that suits your needs, you can pretty much add anyhting 
1: Metro Exodus is an offline game. It doesnât need to send/receive data from a server.
2: Metro Exodus has simple damage values without too many other calculations going on. WoW on the other hand has spells, that have damage calculated based on your stats, effects from those spells ALSO calculated based on your stats and calculations from other players damage/healing spells + environment effects and player locations sent/received from every player as well.
3: Youâre comparing a scripted game with only 1 player affecting the world opposed to an MMO with countless of people that can have an effect on the world.
4: Addons especially that affect the same part of the game (multiple tooltip addons for example) can cause performance degredation.
5: GPU load can be increased by increasing the scaling 
â It also reduces your FPS. So might want to turn off Auto-optimize in Nvidia Geforce Experience and reset Scaling to 100%
The engine has been updated several times over the course of the years; If you want them to create an entirely new engine, that is a process that will take several years. So itâs better to optimize the current engine than working on a new engine that will have itâs own defects and limitations.
Vanilla WoW engine is an entirely different beast by now; That is years of iterations back, itâs hardly the same thing.
Call of Duty is your example? An entirely different type of game with the same engine but basically releasing reskinned games with the same engine?
For my case I didnât use Geforce Experience to adjust my settings.
I have AA disabled, MSAA disabled, Post Process AA disabled, Resolution Scale set to %100, rest maxed and still 40-45 FPS in raids. Canât even do proper PVP
If your settings are alright then itâs most likely the addons.
Try disabling the following:
Scrolling Combat Text addon (MSBT/Pitbull)
Nameplate addon (Plater?)
Weakauras
There is so much going on in that screenshot, tons of people using spells and such where addons will definitely start to cause issues.
Here is what you should do, remove all addons, install one addon at a time and see what the difference will be. Itâs obviously the addons thatâs causing this, some of the addons might even be out-of-date which can cause FPS problems because of LUA-errors
You DO know that expecting a game engine, that was designed before even 1440p was mainstream, to run 4K is asking a bit much�
Factor in that many 2080 cards will not be overly happy with 4K at much over 60Hz & youâre looking at a relatively âbad timeâ.
Your CPU may also not be entirely happy to draw frame as fast as your 2080 can process them; use HWiNFO64 to monitor the relative load on your CPU & GPU; if your GPU is all but âfalling asleepâ, while your CPU is bursting blood vessels, then you have part of your issue solved.
Generally speaking, there comes a point past which throwing expensive hardware at WoW is a waste of cash⌠1440p & ~144Hz is where I think most would find that outlay to performance is capped.