Use Vulkan for the best performance on modern hardware.
Wii U games have different keys for different regions (USA, EU, JPN). Ensure your key matches the region of the game file you downloaded. cemu wii u title keys full
On Windows, if you have "Hide extensions for known file types" enabled, you might accidentally name your file keys.txt.txt . Verify the file properties to ensure it is a standard .txt file. Use Vulkan for the best performance on modern hardware