The European Solar Telescope (EST) will be the biggest solar telescope in Europe and it is currently concluding its preliminary design that includes adaptive optics from its first light. In this phase of the project, the feasibility of the project is demonstrated and the design must meet the requirements after having mitigated the major risks. The top level requirements capture the objectives values for the telescope performance and the analysis verify its fulfilment, including margins for potential factors that are difficult to predict accurately before construction. Optical quality, photon transmission and a few more requirements of the telescope are identified as key performance parameters due to its impact on the top level requirements. The values achieved for these parameters has been estimated for predicting, tracking and managing the telescope performance during the evolution of the detailed design, construction and commissioning. This talk presents the key performance parameters that are proposed among all the top level EST requirements and it reports our approach for its estimation during preliminary design based on simulations with different tools, error budgets and tests on laboratory prototypes. We will also present an introduction to the characterization of the main sources of disturbances: atmospheric seeing, wind and vibrations.