Kubuntu: Launch Steam Big Picture from the Login Screen (For Better Performance on Low-End Machines)
If you don't find what you need here, check out #beholderveekubuntuguides , there might be the thing you're looking for.
If you're on a low-end machine and want to get the most out of it while gaming, but are not willing to sacrifice the convenience of a good desktop environment while doing other things? No problem.
Open this link and just follow the instructions:
Login to Steam Big Picture Mode in Ubuntu