I would be glad to oblige but the umap file is about 120 mo large. If you really want it we can find a way, no pb.
I think you may have a point with the textures. The level contains essentially 45 spheres all textured with a different and very heavy (between 4k and 8k, sometimes more) texture. Could it be the problem ?
It might not be directly related to this setting but it looks like either textures aren’t streaming or they are taking really long to strea. The shadows might be the result of lightmaps not loading either. Try using dynamic lighting or if you are already using that, try using lightmaps with different resolutions as a test. You could also run some tests with and without the objects, changing the scalability texture quality and so on.
You could also leave the window opened and check if everything loads eventually, at least to narrow down the issue.
I removed half of the spheres and everything is ok.
The texture streaming was on (when i turned it off the lighting went wrong even in the viewport) but obviously i was asking too much.
The textures are technically textures but functionnaly they are supposed to be nice and detailed pictures, that explains the unreasonnable amount of 8k^^
I guess i have learned my lesson. Everything’s clear now (aah yes, lightmaps ARE textures !!).
Again, thank you very much for your time, your help and your explanations, Dissonanceint, very appreciated.