I've heard the same thing about DirectX 12 vs DirectX 11. One book basically said that you will probably have worse performance in DirectX 12 vs DirectX 11 if you don't know what you're doing.
DirectX 12 only gets interesting when you want hardware raytracing support to make use of the new Nvidia cards on windows. Tbf that is pretty cool, which is why I actually dabbled with it a little. But it's not necessary for the vast, vast majority of graphics applications.