Same problem here! We have solved in the same way: using the “Use Separate Process” options, and since we have many sublevel to load we set around 5sec of deley pre-warm up and 300 frames of warm up, but we hadn’t to wait anything more than the time to open the separate process indeed (and everything worked perfectly)!
If someone from epic is reading here I’d like to add a point: it happened many times the exactly same thing but when I packed another project! To be more precise: in the editor everything was in good quality, even trying the New Editor Preview there weren’t any problem, but as soon as I tried the Standalone Preview or the .exe after the package the scene was loaded as if was in low quality (exactly as in DaBoss’ screen); the only way we solved was lowering the lightmaps of static meshes in the scene (hoping it doesn’t low too much the quality of the scene); through the GPU profiler I found HZBO was taking ages to be calculated, but even tring to disable it and any other AO the problem continued and the GPU profile was saying a sublevel of “Base pass” was taking ages to be calculated; short story end surfing the web, finding few pages with high technical discussion in which I wasn’t even understanding a word, and solved, as I said, lowering all the lightmaps!