Through shifting the rows adjacent to the line-defect along the waveguide direction, slow light photonic crystal slab waveguides with electro-optic polymer filled holes that show average group indices of 123 and 61.5 are obtained by three-dimensional plane-wave expansion method calculations. It is shown that the slow light properties and the buffering performance are enhanced by using an efficient method based on retreating the anti-crossing point with enlarging the lattice constant. This method has been shown to improve not only the bandwidth and flatten dispersion but also to reduce the variations in slow light properties that could occur due to fabrication inaccuracies. The performance of electro-optic modulation is drastically enhanced by exploiting local field enhancement induced by slow light effect. The buffering performance of the photonic crystal based buffer configurations are investigated and compared in terms of application needs. Since the modulation sensitivities of center wavelength and delay time change linearly with the applied voltage while remaining the buffer capacity and bit length almost constant, the investigated photonic crystal structures show promise for flexible and convenient buffering application in optical communication systems.