[Edi]

Especially the detail level is important. Turn off all the grass effects on battlefield, as that is the biggest culprit. Depending on detail level, Dominions will draw every blade of grass up to a certain distance, even though you would need a 10k x 10k resolution monitor to even see them. And if it draws it to a distance of 10 km, as it does on the aptly named "Too high" detail setting, well, your graphics card is not going to like it. Even the lesser levels can easily drive it to exhaustion.