Transaction Isolation level different threads massive number of Writes
An ORM requires information about the first write (SCOPE_IDENTITY or such) to complete the second write. This means 2 (with an OUTPUT clause) or 3 database (with SELECT SCOPE_IDENTITY) calls in general in a client side transaction.

No amount of tinkering with isolation levels will eliminate these 2 or 3 calls.

If you want more performance, then the best concept stored procedure to do an atomic write (in a transaction) to both tables in one database call. This means 50% or 66% reduction in database calls

Going further, you have to decide what "superfast" really means 

For example, should you be using SSDs for your transaction log files: the drive hosting the log files determines overall write speed because of write ahead logging.

Then there the design: are you using IDENTITY columns or GUIDs or Hibernate style nvarchar keys? Has the ORM designed the database for you?