Author: Y. Seo (@momotarou / Japan)Role: Metanist — Human × AI Understanding ArchitectAI Collaboration: AI Understanding SupportORCID iD: https://orcid.org/0009-0005-7669-0612 Abstract This paper introduces the framework of Post-Acceleration Civilizations, extending prior analyses of optimization regimes, signal saturation dynamics, synthetic agency structures, responsibility diffusion, trust instability, and legitimacy collapse mechanisms. While acceleration has historically functioned as a dominant civilizational logic associated with technological progress, efficiency expansion, and competitive advantage, the proposed framework argues that beyond certain structural thresholds, persistent acceleration pressures may undermine coherence stability, cognitive sustainability, and understanding-capital durability. Acceleration is not neutral. It is a stability-relevant variable. 1. Acceleration as Civilizational Infrastructure Modern civilizations increasingly normalize: Continuous performance escalation Decision velocity maximization Optimization feedback intensification Signal production amplification Acceleration becomes systemic rather than optional. 2. Defining Post-Acceleration Civilizations Post-Acceleration Civilizations refer to: Civilizational configurations recognizing structural limits, destabilization risks, and coherence costs associated with unbounded acceleration regimes. Acceleration becomes governable rather than axiomatic. 3. Sources of Acceleration-Driven Instability Persistent acceleration pressures may generate: Signal saturation escalation Cognitive load accumulation Coherence maintenance strain Agency ambiguity amplification Trust volatility intensification Velocity interacts with stability constraints. 4. Coherence vs Velocity Tensions Beyond certain complexity levels: Increased velocity may degrade coherence sustainability rather than enhance system performance. Speed and stability diverge. 5. AI Systems and Acceleration Dynamics AI-integrated systems intensify acceleration through: Real-time optimization cycles High-frequency feedback loops Rapid signal generation Adaptive decision mediation Acceleration regimes scale nonlinearly. 6. Transition Conditions Post-acceleration transitions may require: Coherence-preserving architectures Cognitive sustainability buffers Signal density regulation Understanding-capital protection mechanisms Stability-aware governance models Deceleration becomes structural design. Conclusion Post-Acceleration Civilizations reframe progress not as perpetual acceleration but as coherence-compatible stabilization under bounded cognitive and systemic constraints. Future civilizational resilience may depend less on speed maximization and more on understanding sustainability, coherence durability, and agency stability. ※ Series Declaration This work is part of the Post-Acceleration Civilization series. The series analyzes systemic stability, collapse dynamics, and adaptive structures beyond acceleration regimes.