Photonic crystal waveguide (PCW), due to its large bandwidth, small size, and diversified design of the waveguide structures, represents a breakthrough in the field of optical buffer. Here, a nested structure is designed to improve delay-bandwidth product with ferrite magnetic fluid infiltrated photonic crystal coupled-cavity waveguides. Specifically, three types of coupled cavities, i.e., horizontal-, vertical-, and cross-defect cavity are studied with different concentrations of magnetic fluid. Overall, this aspect of the work is interesting from the perspective of coupled defect waveguides design, which is different from previous line-defect waveguides. Plane wave expansion simulations on the slow light property of the three PCWs reveal that the group velocities are all three orders of magnitude smaller than the speed of light in vacuum and that the corresponding normalized delay-bandwidth product (NDBP) values are all <0.35, with 0.41 being the highest value achieved in vertical-defect coupled cavity. Interestingly, the quality factor (Q), normalized bandwidth (Δω), and NDBP can be readily tuned by varying the concentrations of magnetic fluid. Moreover, all the three coupled-cavity waveguides can be reconfigured, which accordingly changes the corresponding slow light property. Compared to previous works, the results of this work have further improved the slow light property in slow light devices and realized the tunable slow light.