Security and bug fixes have made Plasma 6 run better for me. Wayland support is better now too (which matters to me). Minor features that improve usability. Newer kernel means i can use newer features, which some of the apps I use depend on. The main thing with your setup I was surprised about is that it isn't an LTS kernel (from what I can tell). If you are just not updating and not using LTS software (i can't tell), then you are missing plenty of security fixes.