Encountered the same issue today. Updated from 16.04 to 18.04 on a SuperMicro server with ASPEED AST 2400 VGA controller. The boot console scrolling was very slow after the update and X11 performance was unusable. Enabled the "proposed" apt feed and updated to 4.15.0-44-generic kernel. Can confirm that console performance is back to normal again.
Encountered the same issue today. Updated from 16.04 to 18.04 on a SuperMicro server with ASPEED AST 2400 VGA controller. The boot console scrolling was very slow after the update and X11 performance was unusable. Enabled the "proposed" apt feed and updated to 4.15.0-44-generic kernel. Can confirm that console performance is back to normal again.