Sorry gents my comment about Value2 should have read Value3 which is not part of the Primary key or any other key and is probably a red herring although I...

I understand your point about parallelism not being the issue but the evidence does indicate it is.  Anyway for information, the table definitions are:-

CREATE TABLE [Table1] (

 [Field1] [nvarchar] (10) COLLATE...

I don't understand why the table structure would have any impact?

Suffice it to say that Table1 has a primary COMPOUND key on Field1+Field2.

It's worth noting that If we replace...