i see, why is it designed that way, was there a reason for this?
The current method of adding the detail normal map on top of albedo was probably chosen out of simplicity, rather than performance. It's a bit cheaper for sure, but not enough to make a difference on modern GPUs (even on mobile). That said, its downsides aren't that noticeable in actual gameplay in the first place :)
For reference, it's been this way since detail mapping was added to SpatialMaterial in Godot 3.0.
