The most common cause of slow scene loading is a poor internet connection. Check to make sure your internet connection is stable and running as expected (use a site like www.speedtest.net from the device itself). A ‘normal’ result is above 8Mbps on 4G, or above 3Mbps on ADSL.
The second most common cause is trying to load too much content at once. Check any images you have uploaded to make sure they aren’t too large (for example a small button does not need to be 1000px - 250px may work just as well if the user never gets close to it).
Also check your 3D models - if the zip file you are uploading is over 7MB or so, it may download slowly for your end users. We do compress any textures you provide, but if the model file itself is large (due to lots of polygons/triangles) it will download slowly. Ask your 3D model supplier if they can simplify the model.
Generally we find if a model is big enough to download slowly, it also has too many triangles to run smoothly on a device, so simplifying the model will greatly improve your users experience.
Another option is to break up your single scene into multiple, smaller scenes. You can then either provide your users with more triggers, or link to those scenes using a scene button. This will break the loads up, making them less noticeable for your user.