Viking: Battle for Asgard has a big performance issue which completely ruins the game experience. However, this can be easily fixed(surprisingly so).

If you don't like reading you can watch my video guide.

Steps:

1. Run your game from steam and select "Configure Game..." when prompted
   
2. Go to "Graphics Configuration"
   
3. Under the "Visual Effects Quality" tab select a lower setting.

You should now have stable 30 frames per second during gameplay. For some reason this setting was causing most performance issues in the game. Even other settings like "Antialiasing" or "Shadow Quality" didn't hurt the framerate as much. I'm assuming this was a coding blunder when the game was being ported to PC(it was originally released only for Xbox 360)