If is the same quantity of elements you use in both there is no difference in performance diference. If after combine in 3D software you optimize it it can be better but combining meshes in 3d software and made it in ue4 changing nothing there is the some quantity of vertex, tris, materials etc etc. The difference realy is was in that in 3d software you can optimize a big scene in ue4 direcly you cant. But you still export the final mesh you combined in ue4 by using export fbx file of this asset and optimize it in your 3d software then bring it back to ue4 it can help if you want to do it.