My test case turned out to have some errors in it, but the effect persists even after these are fixed. In fact, you can run some of the better test cases, like Sascha Willems Vulkan code at
GitHub - SaschaWillems/Vulkan: Examples and demos for the new Vulkan API
On my NVidia 1060, the instancing test case runs at 1900 fps. On the Radeon 270X, it runs at 240 fps. This is a much slower performance than it should have, and slower than it did have on the Apr 19 beta.
I have no idea when the performance changed. I was using the beta for months and then upgraded only recently when a friend told me the production drivers now handled Vulkan.