So, I have this problem with our company's server. It is working like a charm since the 2011 with its very first OS installed, Ubuntu 11 Oneiric Ocelot.
During these years, it has been upgraded to the newer version, passing from the 12 standard series to the 14 LTS --> 16 LTS --> 18.04 LTS.
Well, there's a problem exactly on this last version. It may seem stupid, and I did not consider it at all, but it is annoying.
So, after booting Ubuntu, I am welcomed by the login page, and there I get a huge FPS drop, like 1-2 FPS, making the mouse unusable. Once I drop in the password, everything starts working fine, and the system becomes perfectly usable.
It happens at the first boot only. If you lock the screen during a session, the problem does not reappear.
I think there's some problem with the pre loaded VGA driver. In fact, after logging in, the screen goes black for 1-2 seconds and, then, it reappears with the GUI at normal speed.
Any advices? VGA Hardware is
(lspci -k | grep -EA3 'VGA|3D|Display')
Subsystem: Dell MGA G200eW WPCM450 Kernel driver in use: mgag200 Kernel modules: mgag200