To foster the adoption of active disturbance rejection control (ADRC) and support its deployment even on low-cost embedded systems, this article introduces the most efficient implementation of linear discrete-time ADRC to date. While maintaining all features and the exact performance characteristics of the state-space form, computational efforts and storage requirements are reduced to a minimum compared to all existing implementations. This opens up new possibilities to use ADRC in applications with high sample rates, tight timing constraints, or low computational power.