I'm reluctant to assume that game optimization for SLI is the issue, because you state that this issue is consistent across multiple games.
Your only option is to troubleshoot, and by process of elimination determine what is causing the issue.
You can try disabling SLI and test each GPU individually. This should determine whether or not the issue is with SLI. If performance isn't normal with one GPU installed, try seating the GPU in an alternate PCI-E x16 slot. That would rule out SLI not working correctly and/or a defective motherboard slot - perhaps even a defective GPU.
You can also try creating a partition on your storage drive with a fresh Windows, or re-installing Windows to ensure that other software isn't the problem. Have a clean install with nothing else downloaded except for the latest Geforce drivers, some GPU monitoring utilities, and a GPU bench-marking utility like Unigen Heaven Benchmark - or whichever you prefer. Test the GPU's separately and then together.
If the cards work normally separate but not together, the issue could be the SLI bridge, or perhaps the motherboard. Using a different SLI bridge connector would rule out defects with the bridge, and testing in a different motherboard would rule that out.