Quantum coherence in diamond-based NV centers, crucial for quantum computing applications, faces limitations from decoherence effects. This study examines how nitrogen impurities (P1 centers) impact NV-spin coherence using cluster correlation expansion and density functional theory. Results show T2 varies linearly with P1 concentrations (log scale, -1.06 slope), matching experimental data. The Jahn-Teller effect and hyperfine interactions significantly influence decoherence dynamics. These findings establish theoretical T2 limits across P1 densities, guiding material optimization for quantum devices.