I agree with that point, computing lighmass in preview mode takes too much time.
Unreal 4 is targeting top notch graphics towards powerfull PC devices, while Unity keeps running good on older hardware. You can’t have advantages of both worlds, you must choose high visuals Vs high portability.
Epic will need some year before getting better stable.
C++ is faster, and it is not more hard then C# once you learned it (Epic provides easy C++ starter tutorials)