Hello guys i need help… i tried thousands of solutions but nothing helped…
I have :
Lenovo ThinkPad T480 , Resolution: 1366x768 pixels , Processor Intel Core i5 8th Gen 8250U , Base Clock Speed 1.6 GHz , RAM 8GB , Graphics Processor Intel HD Graphics 620 , Dedicated Graphic Memory Type DDR4.
When i play in shadowlands only from the laptop on 1366x768 resolution i don’t have any lag… game is pretty good but when i plug through HDMI my monitor : Samsung 24 Inch * Full HD 1920 x 1080p and i set up the resolution of the laptop same as the monitor and in the game as well i have lag… specially in battle grounds or place with many people… like fps going really low… i can just to level up and that’s it but not BG or Dungeons…
I tried to make my monitor as primary display it helped a bit but not enough , the thing is when i reduce the resolution the game become little bit better if i use windowed mode…
Please if someone can help me i will be very grateful. I tried to find solutions in google or youtube but nothing helped just i realized that the problem are pixels…
Thanks. 
Your GPU doesn’t meet minimum requirements so it makes sense it’ll struggle, especially if you run multiple screens. You probably can’t improve that without a hardware upgrade.
So the only thing is i have to play only from my laptop without the monitor ?
Only thing I can suggest is if you plug another monitor change the settings so it’s the only one active (and not split/duplicate screen).
I did this im using my Samsung monitor as primary monitor , so the display of the laptop is off.
What you can try is running your monitor at a lower resolution. The higher the resolution you’re trying to run at, the harder your graphics card will have to run, and as Chronormi said, the card you have is below the minimum specs for the game, so it isn’t powerful enough to run the game at the full 1920x1080 resolution.
Run WoW at the monitors native res. 1920x1080. Then simply move the resolution scale slider (in wow’s video options - close to where you set the resolution) below 100% till you get acceptable performance.
In this situation a setting of 50% will match the original screens performance. It’s not going to look pretty though.