I already stated that Kubuntu uses Canonical's distro infrastructure (the same way Linux mint does) does't make Kubuntu ubuntu with kde far from it. One of the most important thing about a distro is governance. Its more important than your build system or what package you use. While the governance of ubuntu is run by canonical (who make all the decisions or what goes in and what stays out) that of Kubuntu is run by the community.. It's the reason why Kubuntu choose to stay with wayland rather than go with Mir, its the reason why Kubuntu aligns with kde rather than infuse spyways or caninical branded programs like Ubuntu one on their distro.
As stated comparing Kubuntu to arch is comparing apples to oranges. Your arch runs faster because it was tailored from ground up to your hardware. That is the archway. Kubuntu does it diiferently you have a full featured distros with many components that you hardware may never need. It's ideal for people who care less and don't have the time and patient it takes to run as arch. Heck most distros out there use the Kubuntu model from debian to opensuse to fedora and centos etc.