Find out if transactions have transaction names, and if the contention is caused by transactions with the same name. Try setting the transaction name rule, which forces transactions with the same name to be sent to the replicate one-by-one.
If transactions are not named, you could change the application so that names are added. Then use the name rule to serialize only specified transactions. Suppose a particular type of large transaction always causes problems if the DSI executor threads attempt to process two or more in parallel. By giving the problem transactions the same name, and applying the name rule, you can ensure that the problem transactions are processed serially. Remember, however, that the name rule is applied to all transactions, and all transactions with the same name will be processed serially.