Instancing relatively large Viewport Textures consistently causes frame drops.

**Godot version:**
v3.2.1

**OS/device including version:**
win10, gtx 1080, GLES3

**Issue description:**
Consistent freeze when instancing a scene with a `ViewportTexture` with a high resolution, despite the scene being preloaded.
I also noticed that during these frame drops, the Profiler registers it as being below the average frame rate during them, as if the profiler itself is also freezing.
This happens for every instance and not just the first instance.

**Steps to reproduce:**
create a scene with a 3d viewport, same as this [demo project](https://github.com/godotengine/godot-demo-projects/tree/3.0-d69cc10/viewport/gui_in_3d), set the Viewport size to something like 3500x3500 or higher.

Then create another scene where your previous scene is preloaded into it, and add a control or something to instance the scene into this scene at runtime in order to see the effect it has on framerate.

**Minimal reproduction project:**
I don't want to spend time on one right now in case this is not an issue but just a known limitation in Godot. So let me know and I will.