itch.io

Here's a few tips since you're using Unity.

1) Use Linear color space and not Gamma.
Edit > Project Settings > Player > set "Color Space" to "Linear" (it's Gamma by default)
https://docs.unity3d.com/Manual/LinearRendering-LinearOrGammaWorkflow.html

2) Use deferred rendering when possible for scene cameras.
Select the Camera  > Rendering Path > Deferred 
https://docs.unity3d.com/Manual/RenderTech-DeferredShading.html

3) Checkout occlusion culling if you don't know what it is
https://docs.unity3d.com/Manual/OcclusionCulling.html

4) Batch draw calls
https://docs.unity3d.com/Manual/DrawCallBatching.html

5) General optimisation tips for graphics
https://docs.unity3d.com/Manual/OptimizingGraphicsPerformance.html