A complete circuit-level description of a representative FPGA is presented in this paper, from which a simple RC delay model as a function of architectural and technology parameters is derived. Using this model, the expression for the optimal delay of any path through the FPGA can be formulated. We distill our model into being purely architecture dependent, and use it to capture new insight into how FPGA parameters can directly affect its delay. Several applications of this model are: (1) to gain better intuition of how architecture and process parameters affect the delay path in an FPGA, (2) for initial studies into new circuit designs and integrated circuit technologies, (3) in CAD tools for optimisation and sensitivity analysis. The technique described can be applied to arbitrary circuits, and simulations show that our closed form equations give delay values that are accurate to approximately 10% when compared to HSPICE simulation.