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Unfortunately, the problem lies in the buggy, untested engine. This is one of the weakest aspects of Heroes V. However, I played comfortably on an Acer laptop with Windows Vista Ultimate x64, repeatedly won in online maps on the "Warrior" difficulty level, and reached the "Beasts and Bones" mission in the "Tribes of the East" expansion. I downloaded a huge number of maps – both for the second expansion and for the original game and the first expansion. It was only uncomfortable to play on map sizes of 256x256 and 320x320; otherwise, everything was fine.