The primary cause of performance degradation in transparent VFX like fog is frequently attributed to overdraw. This occurs when multiple transparent surfaces are rendered on top of each other, forcing the GPU to process the same pixel multiple times.

To mitigate this, follow these steps:

1. Optimize Alpha Textures: Ensure textures used for particles minimize empty space. Large transparent borders increase the overdraw cost without adding visual detail.
2. Adjust Particle Geometry: In the Output Particle context of your VFX graph, change the primitive shape from Quad to Octagon. This trims the transparent corners of the particle, reducing the total pixel area rendered per particle.
3. Simplify the Shader Graph: Minimize the instruction count by removing complex lighting calculations. Use an unlit master stack and fake lighting using a Simple Noise node or vertex colors.
4. Enable Low-Resolution Translucency: If using HDRP, navigate to your HDRP Asset and enable the Low-Resolution Translucency pass. This renders high-density transparent elements like fog at a lower resolution buffer before upscaling.
5. Reduce Particle Density: Increase the individual size of your particles in the Set Size block while lowering the Capacity and Spawn Rate. Aim for the same visual volume with fewer overlapping layers to reduce overdraw.

Additional Tips

• Disable Shadow Casting and Shadow Receiving on the Output block to reduce the rendering overhead per particle.
• For URP, implement a custom Scriptable Render Feature to handle downsampled transparency if native low-res passes are unavailable.
• Turn off Sort in the Output settings if the fog density is high enough that per-particle depth order is visually negligible.