It is very important to optimize your projects!
Try to keep your project total size (images + models + audio files + video files) under 10Mb, or even 5Mb.
On your 3D models, do not use oversized textures, and delete all faces that are hidden.
Short loading times and a smooth framerate always make an experience more pleasant for the end user…
On the left hand column, you can see an estimation of the time required to load your project, considering the mobile network speed.
Use as less assets as possible, import them once in the assets library and add them multiple times on the composition.
Don't import oversized images => do not import a 4096x4096 image that will only be used as a small icon!
In the scenes global settings, turn on 'Progressive download', this will allow the project to start as soon as assets used in the starting scene are loaded
Reduce the number of polygons in your 3D modeling software, reduce the size of each texture.
Export your 3D models as .glb and turn on compression in the export settings