When I press preview, then I see GPU performance in task manager goes up to 80-90% but when I render scene my CPU goes to 100% but GPU goes only upto 10-30%, not more than that. Is something wrong in my export settings ? In hardware setting I have selected both GPU and CPU.
My GPU is 1070 8gb and CPU is i7 8th gen,

And Iray is not working with the latest Nvidia Driver (418.81). It always shows Out of memory.
I wasted my two days making it work, then rolled back the driver to 417.71, it started working again.

I know this is a really lame question, but have you rebooted lately?

I find that Iray occasionally complains about "unable to determine" how much memory I have on my GPU, even though GPU-Z shows I have lot of memory available.  Of course it then defaults back to rendering on the CPU.

So...
a) I usually use the "Save MI Scene" method, exporting everything first
b) Doing a reboot fixes the "memory" issue for me, and it will then render on my GPU as it should.

I don't think this is Reallusion's fault, since it seems to be an issue between Iray itself and my GTX 1080.
I recently updated my Nvidia drivers, but haven't done enough Iray work lately to know if that's improved the situation for me or not.

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iClone 7... Character Creator... Substance Designer/Painter... Blender... Audacity...
Desktop (homebuilt) - Windows 10, Ryzen 9 3900x CPU, GTX 1080 GPU (8GB), 32GB RAM, Asus X570 Pro motherboard, 2TB SSD, terabytes of disk space, dual  monitors.
Laptop - Windows 10, MSI GS63VR STEALTH-252, 16GB RAM, GTX 1060 (6GB), 256GB SSD and 1TB HDD