I appreciate the WYSIWYG approach, but it should not take so long to save a simple scene when it takes less time for other 3D environments to save files with similar workloads.

Have a dedicated Adobe Stock folder installed on the machine for 3D assets for the program to reference to avoid such massive loading times (or taking up space in temp storage).

I'm not sure if loading the textures when you open the file would contribute to the load or save times, but I'd put an option to disable it just in case. Create a separate shader view if you decide to add more dedicated original geometry tools to save resources so that users can block out a scene before adding mats and decals.