Changing Database Transaction isolation level to SNAPSHOT for SQL Server replication "distribution" database
With 1 billion changes, I would be worried about the size of TempDB when the row version is added to everything. Hilary Cotter responded to this same question here - https://social.msdn.microsoft.com/Forums/sqlserver/en-US/2edbdc37-c2ed-4c84-ae72-16f89404172b/sql-server-replication-and-enabling-readcommittedsnapshot-on-the-distributor?forum=sqlreplication.

He is definitely the man you want to ask replication related questions to.