For lightmass whose purpose is lightmap baking, I think GPU-based rendering can be slower. Octane, Cycles, etc. are useful in interactive progressive rendering situation but they do not provide good baking solutions. Vray RT 3.0 has baking mode but it is normally slower than Vray CPU mode with irradiance caching because they do not have good noise reduction GPU algorithms. I don’t know much about the internal of current UE4 lightmass, if it can bake scenes in 10mins or sth, i bet it is faster than current naive (path tracing based) GPU-based rendering/baking. But still I would agree there are rooms for GPU baking solutions in the future.