Full scans on the table being updated? Maybe, but I'd be surprised. (Unless the primary key *has* been removed, but I had the impression that PKs were mandatory for Logical Standby...)
But what about other tables (in the standby instance) with FK constraints referring the the table undergoing mass updates? Are their indexes on the FK-constrained columns?
Unindexed foreign-keys can be extraordinarily painful, as I recall...
Anyway, just a thought...
-- Cheers, -- Mark Brinsmead Staff DBA, The Pythian Group http://www.pythian.com/blogs