Nvarchar perform better than varchar?

I am really surprisde to see that in my MS SQL table, I am defining a field varchar and doing group by with another table's field.

When I do the same thing with the column as nvarchar it is 3 second faster than varchar when, theoretically, varchar should be faster because of 1 byte char.

Can anyone explain me why nvarc开发者_如何学运维har is faster in this instance?

Thanks in advance.

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Operating systems use Unicode internally. I think that makes nvarchar faster since it does not need any converting.