Hey guys,
 
I would like say big thank you for great insights here .
 
So I have LG43UD79 3840x2160 with two DEL 27" 2560x1440.
 
I got pretty lag(like 2-3-4 seconds) when I'm on fullscreen and browsing. GPU is between 15% and 40% range, but for some second it could spike up to 75%!
This is with 3 browsers opened and 20 sites on them. No other applicaton or program is running.
 
If I screen split LG on 4 and browsing on just one quarter of the desktop everything seems to work smoothly for some time, but when I start more browsers lag is happened again.
 
Not sure if my PC configuration is important here but it's pretty good expect video card. I was running lots of processes when I was only using two 27" 2560x1440 and never had any lag.
 
I'm with GeForce GTX 660.
I'm using LG with Display Port
 
Is there a chance to know this is 100% video card fault?
Could you help me what could be the problem and solution?
 
Regards!

A spike to 75% usage isn't a problem. How much vram is being allocated? How much system ram are you using? 

What browser & OS ?

Hey, appreciated your help!
I will post screenshots. 
 
System: 

 
LG 4k" :

 
My other two monitors:
  
 
Hm... VRAM is much less on lagging monitor..?
 
I believe is obvious, but my tech skill is it very limited to make any guesses.
 
Browser Chrome,but I've tried everything.
OS : Win 10
 
 

How does it work with Only the 4K monitor on the 660 GPU & not your iGPU ?
 
Try turning "Hardware acceleration" Off in your Browser
 
You are pushing that little GTX 660 very hard
 
How much VRAM do you really need at 1080p, 1440p and 4K?
 
 Nvidia GTX 660 Specs


Time to upgrade your GPU

elrow999
Hey, appreciated your help!
I will post screenshots. 
 
System: 

 
LG 4k" :

 
My other two monitors:
  
 
Hm... VRAM is much less on lagging monitor..?
 
I believe is obvious, but my tech skill is it very limited to make any guesses.
 
Browser Chrome,but I've tried everything.
OS : Win 10
 
 

Looks like your 4k monitor is being pushed by your intel gpu, and not your gtx 660. Make sure you have your 4k monitor plugged into your gtx 660.

Just checked out the max resolution for the gtx 660... and it appears you won't be able to run all three displays off the gtx 660 at full resolution, so you'll definitely need to upgrade your gpu to use all your monitors at full resolution.
 

weeeeeeeeeeeeeeeeee guyssssssssssssss you are awesome !!! 
 
 Yeah ... It was not plugged into a GTX 660  
 
You are 1000% right @Sajin 
 
 I can't get max resolution on 3 displays with GTX 660. When I plug three displays into a GTX 660, one of display maximum resolution is 1920x1080,not 2560x1440,also getting a little bit of lag when 3 of them are running in the same time. 
 
So I will be looking to upgrade my video card, but nonetheless working without lag on two of them at this moment feels great :) 
I don't want to overdo asking for help, but since you know my problem and what I need I will kindly ask if you can recommend me what video card will be optimal for my setup. They Both monitors support Displayport and HDMI.
 
I can't say enough my gratefulness to you!
Cheers!

A GTX 1060 6GB would work great for what you're doing.