Plasma 5.13 is said to have better Kwin performance and many other improvements.
Can it be installed and can it run in Kubuntu 18.04?
The upgrade doesn't sound very abrupt, it should be from 5.12 to 5.13.
Adding the Kubuntu Backports ppa only brought me 5.12.5.
sudo add-apt-repository ppa:kubuntu-ppa/backports
NOTE JULY 2TH 2018:
I DO NOT RECOMMEND INSTALLING IT. AT THE DATE OF THE FIRST POST AND OF THIS EDIT 5.13 IS UNSTABLE ENOUGH FOR ME TO AVOID RECOMMENDING IT - AS I HAVE TESTED IT IN NEON ITSELF MEANWHILE.
At the date of this edit Neon is using Plasma 5.13 on an Ubuntu 16.04-base.