Next generation on-chip light sources require high modulation bandwidth, compact footprint, and efficient power consumption. Plasmon-based sources are able to address the footprint challenge set by both the diffraction limited of light and internal laser physics such as plasmon utilization. However, the high losses, large plasmonic-momentum of these sources hinder efficient light coupling to on-chip waveguides, thus, questioning their usefulness. Here we show that plasmon light sources can be useful devices; they can deliver efficient outcoupling power to on-chip waveguides and are able to surpass modulation speeds set by gain-compression. We find that waveguide-integrated plasmon nanocavity sources allow to transfer about ∼60% of their emission into planar on-chip waveguides, while sustaining a physical small footprint of ∼0.06 μm2. These sources are able to provide output powers of tens of microwatts for microamp-low injection currents and reach milliwatts for higher pump rates. Moreover, the direct modulation bandwidth exceeds that of classical, gain compression-limited on-chip sources by more than 200%. Furthermore, these novel sources feature high power efficiencies (∼1 fJ/bit) enabled by both minuscule electrical capacitance and efficient internal photon utilization. Such strong light-matter interaction devices might allow redesigning photonic circuits that only demand microwatts of signal power in the future.