Northern Trail Outfitters has a contact count of nearly 10 million records.

Journey Builder performs best when the heavy lifting of segmentation happens before contacts are evaluated for entry, not inside the journey itself. Performing large-scale segmentation in Automation Studio (B) offloads CPU-intensive filtering to SQL Query Activities, which are optimized for bulk processing across millions of rows, rather than relying on Journey Builder ' s per-contact entry evaluation engine. Creating a prefiltered, dedicated sendable copy of the source data extension for each journey (D) avoids lock contention and re-evaluation overhead that occurs when multiple journeys share a single large entry source, and it ensures each journey only processes the exact population it needs. Option A is a valid best practice for decision-split performance generally, but Attribute Groups govern contact-level data joins for Contact Data-based splits and does not address entry-source scale at 10 million records specifically — it ' s a secondary optimization, not one of the two primary levers for this scenario. Option C describes filtering directly on the entry source Data Extension, which still forces Journey Builder to evaluate the full underlying population against the filter at entry time, rather than pre-segmenting it, and is less efficient than doing that work in Automation Studio beforehand. Reference: Journey Builder Performance Best Practices, Automation Studio Segmentation.