Heroes of Might and Magic V: Tribes of the East (Steam license) is lagging.

I recently bought a license on Steam and wanted to play. But I encountered a problem with terrible slowdowns and lags. The game and its sound are lagging. The game is running on Windows 10.
Here are links to videos as examples: https://www.youtube.com/watch?v=MHuGdE9q1Lk (the PC specs are in the description)
https://www.youtube.com/watch?v=PBmH4-eZYe8
https://www.youtube.com/watch?v=NIWi0MbqzEM
Please help! I've been struggling with this problem for 2 days and can't fix it.