I've been trying out the 11.3 beta today and I've come across the same issue with the 1080ti that others have found. As per Gwild99's post - I just tried the default La Femme scene with the suggested settings.

1080ti: 20.12 sec (twice as long as Gwild99's 1080)

2080ti: 5.00 sec

2080ti+1080ti: 3.64 sec

With the test scene mentioned above (https://www.renderosity.com/mod/freestuff/benchmark-test-scene-for-superfly-/83640), using the settings as loaded, I got:

GTX 1080Ti: 6600.24 Seconds

RTX 2080Ti: 622.54 Seconds

GTX 1080Ti+RTX1080Ti: 373.8 Seconds

I also tried a render with one of the new included scenes - AJ Futuristic Base 5960X 151.41 sec 1080Ti 208.41 sec 2080Ti 14.70 sec 1080TI+2080Ti 10.73 sec

So as others have found, running a 1080ti with another card seems to get it back up to expected performance, but alone it is worse than a cpu render.

i7 5960X, 32GB RAM, GTX 1080Ti, GTX 980 Ti, Windows 10 Professional.  Running Daz Studio 4.11, Poser 11, Vue Inf 7, Photoshop CS4