This is a video codec error. Ensure Cemuhook is installed correctly. Go to Options > General Settings > Graphics and check "Enable H.264 video decoding".

Dense city areas often saw performance dip below 20 FPS.

Lego City Undercover was a massive hit for the Wii U, but early versions of the game’s digital files (ROMs/ISOs) were notorious for performance issues and technical bugs. If you are looking to revisit this open-world classic via emulation or original hardware, here is everything you need to know about the "fixed" versions and how to ensure a smooth gameplay experience. The Issue: Why a "Fixed" ROM was Needed