1. A design structure embodied in a machine readable storage medium for at least one of designing, manufacturing, and testing a design, the design structure comprising: a computer system comprising a CPU; a storage device; circuitry for providing a speculative access threshold corresponding to a selected percentage of a total number of accesses to the storage device that can be speculatively issued, wherein the speculative access threshold is determined by an equation which includes at least one of (i) a system access request rate and (ii) a percentage of access requests that do not have valid cached data available and require data from the storage device; and circuitry for intermixing demand accesses and speculative accesses in accordance with the speculative access threshold.