If the game is still unplayable (slideshow FPS in caves or rain), you need to modify the game engine itself.
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The latest version (1.3.2) introduced more particle effects for Delta Pokemon encounters. Some players report that version runs significantly faster on potato PCs. pokemon insurgence lag fix
Navigate to C:\Users\[YourName]\Saved Games\Pokemon Insurgence . If the game is still unplayable (slideshow FPS