Heroes 5 + Windows 8.1 + laptop: terrible performance.

I've tried launching it in every way possible, reconfigured the graphics cards and compatibility modes, but the FPS fluctuates wildly from ~150 to 5(!) FPS (when rotating the map), and with any action (selecting an enemy, moving, etc.) or event (start of a battle, capturing a mine, etc.), the FPS drops by 30-80 points. When the game is running, the coolers are making noise as if I were rendering a video. Thanks for the help, but I uninstalled it because I can't play it like this.