This article explains how journal envelopes are processed with Archive Shuttle
Prior to Enterprise Vault 10.0.3 journal envelope information was not returned by the Enterprise Vault API. In versions of Enterprise Vault prior to 10.0.3 a process called ‘Envelope Reconstruction’ is used to rebuild a journal message, where the API-retrieved item can be added, thus giving the entire P1 & P2 message.
Envelope Reconstruction does have a performance impact but is essential for maintaining data integrity and Chain of Custody for journal archive migrations.
In testing the performance impact it was observed that if extract performance was N with an Enterprise Vault 9.0.4 server doing the journal extract (and envelope reconstruction) and that machine was upgraded to 10.0.4, the extract performance would be between 4x and 6x faster than N.