In video games, shadow settings usually affect performance a lot. In many games, lower shadow quality makes the game run much faster. Raise shadow quality, and FPS often drops fast. This happens because shadows need a lot of work every frame.
Shadows are not pictures ready to use. The game creates them new every time the screen updates because lights and objects move while you play. The game must check what blocks the light.
One main way, in many games, is this: the game draws the scene from the light's view. This creates a shadow map that shows where light is blocked. Then it draws your normal view and adds the dark parts from the map.
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Higher shadow quality usually makes more work. Bigger maps often give sharper shadows. Extra layers make the edges softer. The game checks more objects, even far ones. If there are many lights, the work repeats for each light. All this adds drawing and math every frame. The graphics card works harder, and FPS goes down.
For example, in a forest game: low shadows show hard black areas under trees. High shadows often show soft dark areas that change when leaves move. That needs the game to check everything again and again. Or in a room with lamps: each lamp makes its own shadow. High quality usually checks walls, chairs, and people for every lamp. Speed drops quickly in many cases.
Shadows also change how light looks on nearby surfaces. A shadow on the ground often makes walls darker too. This adds extra steps to calculate lighting and color. Some new games use ray tracing for shadows. It follows real light paths. Looks very good but needs much more power.
For comparison, textures are simple. They are pictures on surfaces, like tree bark or clothes. The game loads them once into memory. Then it uses the same picture as things move. Higher texture quality uses more memory space but adds little work per frame if memory is free.
To summarize, shadows need constant new work every frame. That is why they usually affect FPS the most. Lower shadow quality first if your game runs slow.
Source: Reddit