Both contribute — but the open-plan environment’s specific contribution is measurable and physical. The Irrelevant Speech Effect and the orienting response cascade are physiological mechanisms that operate regardless of individual disposition, experience level, or motivation. A sustained environment of 60–70 dB ambient with continuous background speech imposes real cognitive loading on every employee performing focused work in that environment. The physical environment’s contribution to cognitive fatigue is additive to individual factors, and is the most directly and quickly addressable through physical intervention.