Your GPU is maxing out.
File 1:
- Code:
| +- GPU Core : 99 30 99 (/nvidiagpu/0/load/0)
File 2:
- Code:
| +- GPU Core : 99 37 99 (/nvidiagpu/0/load/0)
The numbers are a %. The one on the left is the current GPU load %, the one in the middle is the average, and the one on the right is the maximum that Open Hardware Monitor saw. 99% means your GPU doesn't have anything left to throw at it, and it's slowing down at this time because of that.
Your CPU cores are only at 75% and RAM at 65% so there is absolutely no doubt that you're maxing out your GPU.
With that in mind, you will want to lower your GPU usage. You can do this by lowering some quality settings in favor of performance. If you have all 4 windows set at maximum quality, of course the game is going to lag when you're in the middle of a huge pack of mobs and throwing out a bunch of abilities with flashy effects (on 4 game windows!). Another way is to reduce the size of your main window (you would do this in your Window Layout), because the larger that is, the more GPU power it takes to render the game (not to mention 4 of them!). Note that the smaller windows are all rendering at the same resolution as the main window (1920x1080) even though they are smaller (960x540).
Another thing you can try is to reduce the overall FPS, but the effects of doing so are relatively limited -- FPS limiting works best when the FPS is too high, not too low. But, reducing the foreground FPS from 60 to 30 will free up some CPU and GPU for the background windows, for example, and reducing the background FPS from 30 to 20 will free up some for the foreground window. You can do this using the CPU Strategy Wizard.