1080 & SLI of powerful cards is not the best --> CPU can bottleneck
Max your details
Test in a Window mode @ higher resolution
Set Nvidia Control Panel & Windows to Max performance
Nvidia Guide (excerpts)
If we were to massively simplify the process of creating a frame in an 2-Way SLI system then measure how long each component took to complete its share of the work, it might look like one of the images below. Each individual frame is first prepared by the CPU and then handed off to a GPU to be rendered as illustrated.
As stated before, however, this isn't how things will look all of the time. It's entirely possible that the CPU may only be minimally slower than the GPUs or vice versa, and that is just enough for us to either begin observing lower GPU scaling or under-utilization in additional GPUs. While it's true that additional performance will still be seen under this condition, how much we actually benefit is what becomes the big question, so let's examine a case where a second graphics card is added into a system where the CPU only slightly outpaces the GPU in its tasks:
Introducing a second graphics card to our system has yielded a measurable performance gain of about +15%, however our GPUs have a lot of downtime that wasn't present before. More importantly, our CPU is now constantly busy preparing new frames, and with each GPU completing its frame long before the CPU can provide it with another, we're back to being CPU-limited. If we want to see more use out of our second graphics card, that means we have some tweaking to do.