Thanks for the scene. We can reproduce the problem here.
I think it's caused by the V-Ray version used to compile each Forest release: FP 4.4.1 was compiled with V-Ray 3.1, and FP 5.0.2 with 3.2 and 3.3.
According to the V-Ray release notes, embree support was added in V-Ray 3.2, so probably is not used while rendering with FP 4.4.1
Unfortunately we cannot change that, so i suggest to use Forest 5 with the latest update of V-Ray 3.3.
Testing your scene with 3.30.02, embree build time is acceptable (only a few seconds here). We'll do more tests using large scenes, and with the upcoming V-Ray updates.