Hi Kent,
Well, if you have an application that does select into in a database and that
database has "select into" turned off, the performance is going to be zero.
Turning on "select into" will allow the application to do select intos, and it should now have infinately better performance than before. Not because the select intos aren't logged, but because they are allowed to proceed in the first place.
If you are comparing SELECT INTO against CREATE TABLE + INSERT SELECT, then yes, there would be bettter performance because the inserts aren't being logged.
It is best to avoid turning on DDL IN TRAN. Instead perform the DDL outside of larger transactions. When DDL IN TRAN is on and a transaction performs DDL inside a transaction, exclusive locks are held on various system catalogs that can block other sessions from accessing objects - which can cripple server performance, particularly if the transaction holding the locks is very long. In 12.5.x, the system catalogs are allpages locking, so the locks for one object also lock all the other objects that have entries on the same page.