Low level api like directx are mostly directly used by engine makers. It allows them to support more hardware without coding specifically for them (like it was on PC pre DX) and the apis being similar allows more easily for them to port among different platforms
Save for the actual api implementation being more efficient over the other this shouldn’t be the case. What can happen however is an api to offer more control and with such control the developer to make something that impact it’s performance (It was common on early dx12 and Vulkan days, where despite being lower level some devs would make optimizations that were less efficiency than what nvidia did on a driver level for dx11 for example, so even though developers had in theory more flexibility ot increase performance they ended up losing it)
MS said they want to align new DX versions with xbox releases. So likely AMD and Nvidia will continue to work with Ms to add some custom extensions that will be used by a select number of PC games for a couple years, and then when the next generation of xbox comes it will incorporate them.
Most of the DX12U spec was like this already for example.