VKDoom has optimisations in it for Vulcan which are not available in the base GZDoom. The plan is to eventually roll those optimisations over into GZDoom as well, but VKDoom can perform noticeably better on certain systems at the moment. Take for example the WAD "Escape Pod". My current setup is 64GB 3333MHz RAM, an RTX 4070 Super and an i7-8700k overclocked to 4.8GHz. On base GZDoom at 4K in the opening areas of the map I average about 50fps. With VKDoom it hits and mostly stays at my 4K monitor's VSynced cap of 60Hz/60fps.