that means there is a performance penalty due to the translation, usually around 10-15%.

The perfimpact here would also be there if apple provided VK drivers. The impact comes from a mismatch of the VK features being used VK the hardware. Vk is not realy a write once run anywhere apis the intention is devs optimise for the feature and pipeline tradeoffs of each GPU vendors. Apples GPUs are very different from PC gpus from AMD and Nvidia and taking a VK pipeline written for these and running it on apples gpus (un-modfied) will have a big performance impact (in the case of modern games it will be way more than 10 to 15%).

If apple provided VK drivers games would still need to re-write thier VK engines for apples GPUs to run or they would need to pull in a VK shim later (simlare to MoltenVK) that fakes gpu features and has a Big perf impact. The only VK engines that could run well on apples GPUs are those optimised for mobile android phones and yes dolphin has such an engine but moltenVK has not been written to expose that sub-set of VK so they use the desktop PC vk apis and thus have this impact.