This is an issue that still comes up often, and not everyone reads the forums where problem is tracked as a bug. So I'm adding the instructions here in hope that more people will see it.

Bug description: At this time the Steam Overlay is causing graphical bugs in Battle Realms as well as massive FPS drops in some cases. These manifest as lines through the game, broken unit shadows, game slowdowns and so on. Sources:
Exhibit A, Exhibit B , Exhibit C , Exhibit D
You can disable Steam overlay just for Battle Realms. You do not need to disable it for all games. If your minimap doesn't load. Pressing alt-tab and then returning to the game should make it appear.

1. Click library link in your Steam client
   
2. Right click Battle Realms
   
3. Click properties
   
4. Uncheck "Enable the Steam Overlay while in-game

Click close and you're set to play the game.

Picture guide:


Bugs can be reported here: https://steamhost.cn/steamcommunity_com/app/1025600/discussions/0/2619338771814485619/

Happy gaming and don't forget to join the official BR steam group for the latest news and updates :)