I wrote a custom blueprint node as follows.

Code:
UTextureRenderTargetCube* UDynamicTextureLibrary::SpawnCubeTarget(int32 X, AActor* Actor) 
{ 
UTextureRenderTargetCube* NewTex = NewObject(Actor); 
NewTex->InitAutoFormat(X); return NewTex; 
}
Calling this from a blueprint actor's constructor works and I can use it in a material. However, performance is awful, regardless of the size of X.

Does anyone know why performance is so bad compared to a texturerendertargetcube that is saved in the content browser and how I can spawn them dynamically with improved performance?