GLTFDocument.append_from_scene executed on non-ui thread cost tool much time compared to on ui thread.

### Tested versions

Reproducible in Godot v4.0.4.stable

### System information

Mac & Android & iOS - Godot v4.0.4.stable

### Issue description

try to execute GLTFDocument.append_from_file on ui thread and non-ui thread respectively, the time cost on non-ui thread is much higher than on ui thread. for example, in the minimal reproduction project, append on ui thread time cost: 3436ms,
but append on other thread time cost: 117287ms
Hope to see your replay as soon as possible! Thanks so much!

### Steps to reproduce

run the MRP Main.tscn to check result

### Minimal reproduction project (MRP)

[MinimalProject.zip](https://github.com/user-attachments/files/18088789/MinimalProject.zip)

Start by running the attached MinimalProject.zip and Main.tscn, then compare GLTFDocument.append_from_file on the UI and other threads using the reported timings. Trace the relevant GLTFDocument entry points, including append_from_scene from the issue title, and verify that the non-UI performance discrepancy is explained and addressed.

Written by the indexing model from the issue text.