You could be using more GPU RAM than the GPU actually has available, which will cause D3D to constantly move textures/etc in and out of GPU RAM for you, possibly multiple times a frame. Try reducing your texture resolution drastically and see if the problem goes away.

This is basically how Optimus works: (simplified version) all rendering commands are executed on the NVIDIA, and once the scene is complete the framebuffer is transferred to the Intel for display. If you look at the Optimus white paper, section headed "Optimus Copy Engine" (page 18) you'll get further info.

The theory is that for the transfer of the framebuffer, while it takes extra time, the extra performance from running commands on the NVIDIA outweighs what would otherwise be measurable overhead from it, and the net result is higher overall performance.

In practice however Intel GPUs aren't actually useless any more, and for certain workloads can even run fast enough that just using the Intel on it's own can easily outperform the full NVIDIA/Intel combination. I would guess that since you're measuring the copy engine as such a dominant factor your game has such a workload, and I'd suggest doing some benchmarking using the Intel on it's own to verify if this is indeed the case.

Even if this is the case, you may also get improved performance by just doing an NVIDIA driver update or reinstall. Optimus seems fussy about this, and anecdotally I personally got a significant performance increase on an Optimus laptop by doing this. I haven't analysed it in sufficient detail to determine if it's related to order that the NVIDIA and Intel drivers were installed in, some other system software installed after the original NVIDIA driver causing trouble, or something else.

To the OP have you figured it out yet? If so can you post it here... I'm curious.