A new type of wideband slow light with a large delay bandwidth product in a slab photonic crystal waveguide with a triangular lattice of circular air holes in a silicon-on-insulator substrate based on optofluidic infiltration is demonstrated. It is shown that dispersion engineering through infiltrating optical fluids-with different refractive indices n(1f) and n(2f)--in the first two rows of the air holes innermost to the waveguide results in an improved normalized delay bandwidth product ranging from 0.187 to 0.377 with large bandwidth (12 nm<Δλ<32 nm) and group index (14.20< n(g)< 24.62) around 1550 nm. The nearly zero group velocity dispersion on the order of 10-(20) s(2)/m is achieved in all of the structures. These results are obtained by numerical simulation based on a three-dimensional-plane-wave expansion method.