The package kubuntu-low-fat-settings is very good start, indeed.
If you want some additional manual tweak and fix on slow startup or Youtube slowness, try this (some of them are performed by the above package, others not):
1 - Install KDE backports PPA. The latest releases fixes a lot of memory leaks and 4.10 (and above) is really fast:
sudo apt-add-repository ppa:kubuntu-ppa/backports
2 - Disable Nepomuk file indexing: Go to System Settings > Desktop Search. Uncheck "Enable Nepomuk Semantic Desktop".
3 - Uninstall virtuoso-nepomuk, zeitgeist and gnome-keyring packages:
sudo apt-get purge virtuoso-nepomuk virtuoso-minimal gnome-keyring zeitgeist zeitgeist-core zeitgeist-datahub
4 - Disable Desktop Effects, or at least, disable a lot of them, and in case you still use some, make them faster: System Settings > Desktop Effects, under General tab, select for Animation Speed: "Very Fast" or "Instant".
5 - Disable some KDE services: On System Settings > Startup and Shutdown, select "Service Manager", then on "Startup Services" uncheck and stop:
- Akonadi (if exists, in some desktops it's disabled by default)
- Application menus daemon
- DNS-SD Service Discovery Monitor
- Free Space Notifier
- K Remote Control Daemon
- Nepomuk Search Module
- Remote URL Change Notifier
- Write Daemon
6 - Open Krunner (Alt + F2), click on Settings and uncheck all options you don't need (if you don't use krunner at all, uncheck all).
7 - This last one is always valid, for fasting KDE desktops or not: if you want to watch Youtube fullscreen videos, go to System Settings > Desktop Effect, select Advanced tab and check "Suspend desktop effect for fullscreen windows".
8 - Fix Kmix autostart (will improve startup, but need to be repeated every new release while this is not fixed):
sudo mv /etc/xdg/autostart/pulseaudio-kde.desktop /usr/share/autostart/