Hi,
welcome to ROOT6!
What is the LHEEvent class you are using? Is there a link on the web of its header?
In ROOT6 you have much more functionality and performance than ROOT5. For example, you can generate your LHE events in parallel and write them in parallel with TDataFrame. For example, you can have a look at this post if you are interested: Error in <TTree::Branch>: The pointer specified for event is not of a class known to ROOT (the topic name is admittedly misleading 
)
Cheers,
D