An additional camera rendering a single layer in Perspective projection has large impact on framerate

We have multiple cameras in our scene, most besides the main camera being used for UI. Those are set to orthographic projection.

However, we have one camera set up for rendering specific objects that the player is holding - it’s only set to render a single layer, and the culling is set to Depth Only. Only one object at a time is put into this layer.

When this camera is enabled, our framerate drops by about 10-12 FPS. This is true no matter what layer it is set to render (except “Nothing”). This happens regardless of whether any objects are being rendered by the camera. The framerate loss goes away if the projection is set to Orthographic (although that obviously isn’t a solution).

My understanding is that a camera set up such as this should have a negligible performance impact. Is there anything anyone can tell me about why this might not be the case?

Thanks!

Well, Unity needs to figure out what objects in the scene are visible from this camera. My guess is that Unity is spending a lot of time visiting all your objects and clipping them against the perspective camera frustum. Orthographic might be faster because it’s a simpler test per object. If you have purchased Pro, try using the profiler and see what it says is eating your time.