When rigging a 3D model, lights cause lag because real-time calculations (like dynamic shadows or global illumination) add GPU/CPU load during frequent viewport updates.
Rigging requires constant bone moves and pose tests, while real-time lights recalculate how light interacts with the model’s shape and materials—this combo slows the viewport.
To fix it, disable non-critical light features (soft shadows, GI) in viewport settings or switch lights to "preview" mode to reduce real-time work.