Loading lightmap is slow

Dear,

I attach two screenshot. The left image is right after map loading, I guess before applying lightmap on static models.
And right image is static models after 1~2 secs. These are captured from iOS.

I detoured this problem by delayed fade out but this causes too slow loading. It is also seen on PC but that blink passes so fast that it feels ok.

Is there any other option for this? streaming level is not an option for now due to too many progress is done.
For now, when changing level or restart level, the order is as below

fade out(duration: 0.7sec) → open level → delay(about 1sec) → fade out (duration: 0.7sec)

problem is that delay is varying w.r.t mobile device performance.

Thanks. for reading.

45657-lightmapprob.png

This issue is still actual in 4.17. Did you find any workaround (except extra delays)?