Today I added 2 gtx 1060s to my 2 gtx 1080 ti. Somehow the system got so slow and won't run properly.
With the original 2 gtx 1080 ti my system runs very smoothly.
With the 2 gtx 1060s, when I typed "top" , All I see the CPU working on is nvidia-smi / and nvidia-settings.

Does the nvOC use a lot more resources when different NV cards are being used?
Here's my system config:
Mobo: MSI z270-SLI Plus
CPU: Celeron G3900
Ram: DDR4, 4GB
USB/HDD: 32GB USB thumb stick
GPUs: 2 x 1080 ti, 2 x 1060
I removed 1x 1060 and it runs fine. with 2 1080ti and 1 1060.
I insert the cards starting from by pcie16 (closest to the CPU first), then pcie1 (also starting from closest to the GPU).
However, when the 4th GPU is inserted into the pcie1x slot, the whole system just lags and won't work. I'll take a look again tomorrow. If you guys have any idea please drop some hints =)
Enable "Above 4G" in the bios of your motherboard. If you already had that set and still experience the problem, change the nvidia driver from 387 to 384. Drop a line if you need help about switching the driver.