Functions Reflection function Set the update mode of reflection, reflection brightness, blur, etc. Adjust the reflection screen size Filtering of reflected targets (experimental function) Volume light support Control the shape of the...
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There is this plugin which lets you customize mirror performance aspect. Main thing is CustomRenderRate I think, so you lower fps of mirror only for performance increase.
With builtin mirror, you can lower resolution to acceptable number (1024-2048 is fine for far distance), and select disable pixel light option for mirror which give a good boost.