I have just installed Ubuntu 18.04 on a laptop and it worked perfectly. After installing it on another one (Asus RoG, i7, Nvidia GTX 950M, 16gb RAM), everything worked so poorly.
How it manifests: Whenever I want to drag a window with my mouse, or to write something, or to select an area on the screen, the rendering is so slow.
EDIT1: After doing what pilot6 pointed out, I managed to go past the slowness, but got stuck in an infinite loop at the login screen. Heynnema tried to help me out, but my .Xauthority file was missing.
EDIT2: In the end I gave up and reinstalled it again, but this time I chose something else instead of 'nomodeset' (noapic, nolapic, acpi=off), after some research. That fixed my problem.
