I have the same issue - playing on Dreanor EU
I started having issues in Shadowlands, it was first expansion when I really started playing a lot and engaging with the content so lag was very noticeable. It lagged everywhere after launch, in open world, dungeons, things like looting, casting abilities or using auction house were very sluggish. Due to that I did not even consider raiding.
It went away by itself after about 6 weeks after the launch. Weird thing is that nobody in my guild at the time experienced similar issues.
And I have very fast fiber connection, no issues in other games (really the only other online game that is latency sensitive I play sometimes is Path of Exile, and in that game when you have a lag spike that lasts 400ms you can just die, and it never happened, issues are only in WoW). I figured that it was just another time where Blizzard could not handle load of players after expansion launch, and after those 6 weeks or so I did not have any issues anymore. (But since 9.1 I did not really play much, mostly played AH to pay for sub)
In Dragonflight it got worse. But the weird thing is that I only had issues in capital city and in raids. I did not notice problems in dungeons or open world.
Because of that I had to give up raiding, I could still manage heroic raids, but in mythic I was dragging the guild down, my performance was abysmal due to high latency, and I would also die a lot when defensive abilities would not go off in time.
I spent a lot of time on training dummies and I noticed that in places like Orgrimmar or Zandalar there was no lag and I was doing expected damage on dummies.
While when I was training on dummies in raid or in Veldrakken I would get the lag and performance would go down. It was super frustrating.
Then Plunderstorm came, and I had a lot of free time so I played it a lot, and it was as laggy as the raids, even more laggy. On my road to max renown I was only able to win one match, as when I was engaging other players they would desync on my screen and their character would disappear and appear again, I could not fight people as again abilities were going with a delay and when I was aiming a spell, I would clearly hit the enemy on the screen, but I would not hit them because in reality he was in different place. Again this was super frustrating.
Since I had a lot of free time then, I searched for and tried A LOT of different solutions, none of which worked. They were obvious things like updating your drivers, doing network, gpu or cpu troubleshooting, reinstalling the game with no addons etc., but nothing worked. All the tests were ok, my network and hardware was fine, in game latency was a constant ~20ms, FPS was well over 150, no issues in other games, but still abilities would go off with delay, and that delay was very inconsistent, so you could not get used to it. I would get hit, or not hit the enemy, as they were not shown in correct place on my monitor.
But then I found solution that worked for me. Unfortunately I forgot how it was done, since I tried a lot of different obscure methods.
Solution was to add a rule in Windows Firewall to throttle some kind of Windows Update service, or something like that.
After I implemented that, lag was gone in Plunderstorm, I was able to win few matches in a row, which was impossible previously, no more missing abilities or disappearing enemies.
Unfortunately I was not able to test it in raid or dungeons, as I was not enjoying any class and was mostly playing AH to pay for sub, I did not have gear to test. I was just waiting for new expansion.
I don’t understand how, but that rule I added in windows firewall is gone now, and I do not remember how to add it again.
I hope that it rings a bell for someone and they can point me/us in the right direction, and it helps with lag/delay a lot.
Even though lag in TWW is surely caused by Blizzard, lag is much worse now, and for me it is present everywhere. Places that worked in DF like zones from previous expansions are now lagging hard in TWW.
Lag is a lot worse in 11.0.5, I have lag when looting items, opening crafting orders, using AH, training on dummies, using abilities in open world or delves, literally everything is lagging, but the in game bar shows 20-25ms latency, FPS is over 400… I am running out of patience.
I cannot engage with raids and dungeons because of that.
I hope that someone will know what I was trying to say about the solution with throttling update service and tell us how it is done, it may help alleviate the issue until Blizz fixes their s**t.
The only real options seems to be unsubscribe, uninstall the game and move on.