Class member duplication takes very long

One of my UObject classes has an array

UPROPERTY()
TArray<FGridNode> nodes;

of the simple struct

USTRUCT()
struct FGridNode {
	GENERATED_BODY()

	UPROPERTY()
	FVector pos;

	UPROPERTY()
	bool blocked;

	UPROPERTY()
	int flag;

	UPROPERTY()
	int neighbours[8];
};

In OnConstruction the array is filled with one million elements, nodes.SetNum(1e6);

After hitting Play in Unreal Editor, an instance (the CDO I suppose) of this class gets duplicated and its member nodes gets copied.

This takes very long, about 7s. However, a direct copy of an TArray of this length takes only 30ms.
Where does this difference come from? How can I speed up the starting phase of the game?