Which shell, which OS? Is it within a script or when entering from the console? Have you actually timed your tests? My first reaction to your question was th

Which shell, which OS? Is it within a script or when entering from the console? Have you actually timed your tests? My first reaction to your question was that each command is executed as a separate process and depending on what you are doing, e.g. piping outputs to inputs, using temporary files, other operations could create lots of variances between one run and the next.