As we've come to expect, Doom's Vulkan renderer benefits Radeons and hobbles GeForces a bit. The R9 Fury X rockets into second place in both our average-FPS and 99th-percentile frame-time measures, followed closely by the R9 Fury. Both GTX 1060s fall toward the back of the pack, but the GTX 1060 3GB card takes the switch especially hard. Its average frame rate drops behind the GTX 970's, and its 99th-percentile frame time is also significantly worse than the Maxwell card's.
The 3GB card also happens to have the smallest amount of RAM here. We can't conclusively say that the size of the GTX 1060's 3GB memory pool is the cause of this larger-than-expected performance drop, but it's the only explanation that seems to make sense. That said, the GTX 1060 3GB's frame-time plot doesn't exhibit any large or repeated frame-time spikes, so its performance at least degrades in a way that doesn't harm the user experience much. Of course, the real answer here is to stick with OpenGL if you're a GeForce owner.
Another sign that the GTX 1060 3GB might be struggling because of the amount of RAM onboard is the relatively large amount of time it spends past 16.7 ms in our measures of "badness." The GTX 1060 6GB has no such issues—it spends under a second working on similarly-challenging frames. Even the GTX 970 and its unusual memory configuration fare better. Regardless, the numbers above tell a simple story: GeForce owners shouldn't bother with Vulkan if they want maximum Doom performance, but the Radeon faithful should absolutely enable it.