Isn't phased delivery just slower than building everything at once?

Calendar time can be similar. What changes is the size of the bet at each commitment point. Boehm's spiral model and PMI PMBOK 7 both argue that phasing trades a small amount of overhead at each gate for the option to stop, change direction, or re-prioritise — an option a monolithic plan does not offer.

How small should each phase be?

Small enough that the assumptions it tests can be evaluated within weeks, not quarters. The Standish CHAOS data has consistently found smaller delivery units to have higher success rates, though specific thresholds vary by methodology and source.

Does phased delivery always mean agile?

No. Phased delivery is the broader concept; agile (Scrum, XP, Kanban) is one family of methods that operationalises it. PMBOK 7 and ISO 21502 both describe predictive, iterative, incremental, and hybrid approaches as legitimate ways to phase delivery depending on the level of uncertainty.

Why Phased Custom Builds Reduce Software Risk

An insight on why phased custom builds reduce software risk from CodeCraft Enterprise.