This is more of a workaround than a solution, but you could move the lag to a moment where it is less problematic. Make yourself a loading screen, where you instantiate every one of the offending Nodes. That way, it will look perfectly normal, as loading screens naturally tend to have short freezes.
You could either make it look good, or try to hide them as much as possible. For instance, maybe rendering them in a Viewport that is not seen would be enough to "prime" the Node.