The European Solar Telescope (EST) is a European collaborative project promoted by the European Association for Solar Telescopes (EAST) to build a high resolution 4m class solar telescope in the Canary Islands. Since 2008 to 2011 a Design Study has been developed to define the Conceptual Design of EST. During the Design Study, several preliminary performance simulations have been developed in order to demonstrate the feasibility of the proposed conceptual design and provide preliminary feedback for the defined top-down error budgets. Performance simulations developed during the Design study include structural FE, servo-control of telescope main axes, CFD analyses of wind load on the telescope, CFD analyses of local seeing, transient thermal analysis of the telescope structure and environment and AO performance. This paper presents a description of the conceptual design of EST and the results of the preliminary performance simulations, mainly focused to obtain the best telescope image quality.