Phase randomization is essential for the security of practical decoy-state quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources, which are either actively phase-randomized, or gain-switched. However, at high repetition rates, these show correlations, which can ultimately compromise security and performance. We present a 1.25 GHz phase-randomized QKD source based on a super-luminescent light emitting diode (SLED) operating in the C-band as a compact and cost-effective alternative. The source generates ∼100 ps optical pulses with 400 ps pulse-to-pulse separation, compatible with high-speed time-bin encoding. Interferometric measurements demonstrate >99% visibility between adjacent time bins, confirming strong first-order coherence within the same quantum signals, while the spontaneous-emission-driven nature of the SLED drives intrinsic global phase randomization between adjacent signals with a visibility upper bound of the order of 2%. This work establishes a scalable SLED-based platform for high-speed prepare-and-measure QKD systems.