Ubuntu 18.04 poses two problems for Intel + Nvidia mixed graphics users: the power consumption increases when the discrete GPU is turned off, and it is not possible to switch between power profiles through a simple logout (restart is currently required).
These issues were caused by logins, changes to the Nvidia driver packaging (now more elaborate), and the migration from LightDM to GDM3. These issues have been fixed in Ubuntu 18.10 Cosmic Cuttlefish, and these fixes have been backported to Ubuntu 18.04 And can be used. Used for testing in the suggested repository.
If you want to help test these fixes, Enable Suggested repositories (make sure you are familiar with this, because the suggested repositories may cause instability!), Then follow Error descriptionAnd then post your findings here. You will need a laptop with an Intel + Nvidia hybrid GPU and tested with 390 Nvidia drivers. Please note that these fixes only apply to GDM3. LightDM and SDDM still need some work, and “will be provided in the next update.”