Minimize the game using the "Start" key, go to the task manager, find the heroes4.exe process - right-click - Set affinity - check only CPU 0.
Or, to automate this, open the game shortcut properties, in the Target field, add the following at the beginning:
cmd.exe /C start "Heroes4" /affinity 1 (SPACE) "C:/Heroes etc. (i.e., after the space, enter your game directory)
I also recommend a setting to eliminate all animation slowdowns:
Control Panel - NVIDIA - Adjust image scaling - Disable scaling and choose GPU instead of Display.