Kubuntu uses the Plasma desktop, which comes from the KDE community. This interface has the distinction of providing arguably the most customizable experience found on any desktop.
Kubuntu is a great option for people who love Plasma but want access to all the software support that Ubuntu has. Kubuntu is a great way to experience Plasma in a “vanilla” fashion, one with very few changes from what the original KDE developers provide.
While the Plasma desktop has a reputation for being heavy, newer releases are significantly lighter. You may find it to be faster than default Ubuntu.