groggoggrog: Civ IV was using 100% GPU which was quite ridiculous for such an old game. It turns out the frame rate was uncapped. I can only guess what ungodly frame rates the game was running at. So I looked at the game's readme - who does that well in desperation I did - and found you can limit the frame rate. I set it to 60fps and now my GPU usage dropped to 35% at 1080p max settings on a Nvidia GT 750M.

INI File
C:\Users\username\Documents\My Games\Civ4\CivilizationIV.ini

Advanced users can change various Civilization IV: Beyond the Sword settings and options by editing your INI file. To locate the INI files, navigate to "My Documents", "My Games", "Beyond the Sword" and open the “CivilizationIV.ini” file. This file is a standard text files that can be edited with Notepad (included with Windows®).
Civilization4.ini holds the values of all user-controllable options.

ScreenHeight Height Resolution
ScreenWidth Width Resolution
MouseScrolling Allow Mouse Scrolling in Windowed Mode
QuickStart Immediately launch in game
PlayMusic Toggle Music
FullScreen Toggle Window Mode
NoMovies Toggle whether movies play
AutoSaveInterval Set how often autosaves get created
AudioEnable Toggle All Audio
Language Set the game to a specific language
Mod Set the game to load a specific mod

; Set max frame rate clamp (0 means none)
SetMaxFrameRate = 60

; Set to 1 for no intro movie
NoIntroMovie = 1