Hello everybody,
 
I had a stable system running for quite a while, meaning that everything worked as expected, great performance, no latencies, no display errors etc. The setup consisted of a GTX 680, a main monitor BenQ XL2411T running at 144 Hz at 1920x1080 plugged in to the video card's first DVI port, and a secondary monitor Samsung SyncMaster 2433BW running at 60 Hz at 1920x1200 plugged in to the video card's second DVI port. Despite the different resolutions and refresh rates, Windows 7 Pro 64 bit handled everything well, also in games. The drivers I used were always up to date until version 372.70, which was - I believe - the last one before Geforce Experience 3 was shipped.
 
Then I bought an EVGA GTX 1070 last week and replaced the old card. The new card has only one DVI port, one HDMI port and several DisplayPorts. So I plugged in the primary monitor to the DVI port and the secondary monitor to the HDMI port using a DVI to HDMI adapter. (DVI to DisplayPort cables are on their way, but have not yet arrived. Will test with them asap.) After the first boot, the card was not correctly detected, so I downloaded the latest drivers (version 373.06) and performed a clean installation. After reboot, the card was detected properly, Aero working, games fast, life was good. Until the machine was running for a longer time, maybe 2 hours, then the entire system became slow and unresponsive. This would continue when running GPU-heavy applications such as games. The task manager showed that Windows' Desktop Window Manager was causing the trouble, using between 20 and 30 % of CPU computation time, raising even higher when dragging around windows.
 
I could not track down yet what causes this behavior to start, it simply happens after one or two hours of system runtime. And not only when idling, but also when playing games, for example. In the latter case, I also get the "Do you want to disable visual effects?" message.
 
What I can tell you, however, is how to stop it:
- Kill dwm.exe
- Disable desktop composition
- Disable Aero
- Unplug the second monitor
 
If I do any of the above, the CPU load drops to 0 % immediately, but as soon as I revert the changes, it rises again. Restarting the system also helps, but only for one or two hours, as before. So the obvious thing would be to simply disable Aero, but I don't want to do that just to fix an error that was not there before. I like Aero. Instead, I want to fix the cause of the error, but I'm puzzled what it could be. Of course, I checked the system temperatures, everything is running below 40 °C. I also shut down all processes and services not needed, but this also has no effect. I set the refresh rate of my primary monitor to 60 Hz and the resolution of the secondary monitor to Full-HD so that both monitors are equal, with no effect. I updated the video drivers to the newest version 375.57, with no effect.
 
To summarize:
- Dual monitor setup
- Changed my GTX 680 to a GTX 1070
- Updated the video driver from 372.70 to 373.06
- Updated Geforce Experience to 3.0
- Now, the DWM is significantly impairing the system performance both in idle and under load after running for a while
- Unplugging the second monitor or disabling DWM/Aero/Desktop composition temporarily fixes the problem
 
What now? I doubt very much that this is a vendor-specific problem, but asking in this forum still seems like the best place for this question. Maybe it is a known issue? I'm thankful for any hint and suggestion.
 
Regards,
Raoul

Sadly, this did not change anything. I tried both 372.70 and 372.90, but the problem remains.

I followed the instruction and the problem remains. Meanwhile, the DVI to DisplayPort cables arrived, so I use them now, but the problem persists. I have found out that the high CPU consumption also starts immediately when I start Battlefield 1, but not when I start other (older) games. Windows will then tell me that the performance might increase if I disable visual styles. If I don't disable them, everything is horrible again and also stays this way after closing Battlefield 1.

Do you still have your old 680?