Despite two decades of work on decoherence and entanglement robustness, no quantitative principle explains why equally entangled quantum states decohere at radically different rates under identical noise. Here we identify the missing variable: a single integer D∗that measures the minimum geometric distance of quantum coherence in Hilbert space. We show that this spectral bottleneck controls irreversible entropy production, while entanglement depth, purity, and coherence norms do not. The principle emerges from an exact spectral decomposition of coherence under local noise, combined with a controlled asymptotic expansion of von Neumann entropy. States with identical entanglement but different D∗exhibit paramet-rically different collapse times—explaining the stark contrast between fragile GHZ states (D∗= n, fast collapse) and robust cluster states (D∗= 1, slow collapse). This upgrades previous empirical observations to a derived theorem, with a prefactor Acomputed exactly from the Kubo–Mori kernel rather than fitted. The spectral bottleneck is not a diagnostic but a control variable: it determines when classical statistics emerge from quantum superposition. This principle resolves a foundational gap in decoherence theory and identifies the correct macroscopic variable governing the quantum-to-classical transition under local noise.