We introduce a plasmonic waveguide system, which supports a subwavelength broadband slow-light guided mode with a tunable slowdown factor at a given wavelength. The system consists of a metal-dielectric-metal (MDM) waveguide side-coupled to a periodic array of MDM stub resonators. The slowdown factor at a given wavelength can be tuned by adjusting the geometrical parameters of the system. In addition, there is a trade-off between the slowdown factor and the propagation length of the supported optical mode. Finally, we show that light can be coupled efficiently from a conventional MDM waveguide to the plasmonic waveguide system.