KDE.org promotes Plasma Desktop as "fast and efficient" (see the following screenshot), but there is no relevant information given in order to have such experience.
Fast & Efficient
Plasma is faster and more efficent than ever, using fewer resources than the previous generation desktop. -- Relevant text from screenshot
KDE documentation includes several wiki sites, but none of these had any relevant information. Although KDE UserBase Wiki for KWin has mentioned the hardware requirements for the window manager counterpart, that doesn't tell much for the desktop environment itself.
KWin supports compositing, which means using 3D effects for handling your windows [...] It’s hardware requirements are very modest and pretty much every reasonably modern system since 2002 supports it.
Even Kubuntu (KDE-based distro) has its dedicated article on Wikipedia that mentioned system requirements, the article has been revised to note that "not in citation given" for the quoted source. See dated revision of the source via Wayback Machine and one will notice that it was loosely mentioned and poorly quoted.
Above all, what is the official system requirements for KDE?