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Coupled resonator optical waveguides (CROWs) are widely considered promising building blocks for the on-chip processing of wideband data-streams. The performances of these structures are much sensitive to disorder effects, affecting both losses and the maximum achievable slowing ratio. To date, how and how much disorder influences the slow-wave propagation regime is still an open issue. In this contribution, an analytical model is presented, showing the relation between the back-reflected power due to random variations in the resonant frequencies of the CROW resonators, referred to as phase disorder, and the actual group velocity of the disordered structure. The model can be applied to any CROW, independently of the architecture and technology, and is supported by numerical simulations and experimental results. The effects of an increasing phase disorder were observed in the frequency response of ring-resonator (RR) CROWs, where the resonance frequency of each RR is independently controlled by a thermal activation of the optical waveguide.