Mirrors and frame rate in VRChat worlds

Short answer: a mirror makes the scene render again, once per eye. Leaving one on permanently is the single most common reason a VRChat world runs badly. Keep mirrors, but keep them off by default and let players switch on the one they need.

Why is a mirror so expensive?

A mirror is not a texture. It is a second camera rendering your scene from the reflected point of view — and in VR, once per eye. A world that renders at 90 fps with no mirror is rendering three times per frame with one mirror visible: left eye, right eye, and the reflection for each.

The cost scales with what the mirror can see. A mirror facing an empty wall is cheap. A mirror facing your entire main room, with every avatar in it, is not.

What actually reduces the cost

In rough order of effect:

1. Off by default. A mirror nobody switched on costs nothing.
2. Fewer layers. A mirror that reflects only avatars and players skips your whole environment. This is usually the biggest single win after switching it off.
3. Lower resolution. Half resolution is often indistinguishable in VR for a mirror used to check an avatar.
4. Smaller mirrors, facing less. Geometry matters: what the mirror can see is what it has to render.

There is no setting that makes a mirror free. Anyone claiming otherwise is describing one of the four points above.

Should mirrors be on when a player arrives?

No. It is the most expensive possible default, and most visitors never use a mirror at all. Give them a visible switch instead — a button on the frame, or an interaction on the mirror itself.

The exception is an avatar-testing world, where the mirror is the content.

Does this differ on Quest?

Yes, considerably. A standalone headset has a fraction of the GPU budget of a PC, and mirrors are the fastest way to exhaust it. If your world ships to both platforms, treat Quest as the constraint that decides the defaults: off, low resolution, minimal layers.

Doing it without writing Udon

FOS Mirror is built for exactly this. One manager scans the scene, finds every VRChat mirror and takes control of it. Mirrors switch on by distance, by a UI button, or by player interaction, and an exclusive mode guarantees that only one is ever on at a time.

Two details that matter in practice:

• It switches the VRC_MirrorReflection component, never the GameObject. Disabling the object would re-run OnEnable on everything beneath it, Udon scripts included, and would hide the decorative frame along with the reflection.
• The per-mirror component does no per-frame work. The manager owns the single loop, so adding mirrors does not add update cost.

The pack is free and requires only FOS Essentials Core.