 <p>First, I don't think that setting <code>$MaxPrecision</code> and/or <code>$MachinePrecision</code> will have the effect that you desire. Here's an example.</p> <pre><code>$MaxPrecision = $MinPrecision = $MachinePrecision; result1 = Nest[4 # (1 - #) &amp;, 0.123, 10^6]; // Timing result2 = Nest[4 # (1 - #) &amp;, SetPrecision[0.123, $MinPrecision], 10^6]; // Timing </code></pre> <p>On my machine, the first computation takes 8 hundreths of a second, while the second computation takes nearly three seconds. The reality is that the first computation is done in machine arithmetic, while the second is done in software arithmetic, that happens to have the same numerical precision.</p> <p>I think that the easiest way to force machine arithmetic in Mathematica 8 is to use <code>Compile</code> with the <code>"CatchMachineOverflow"</code> and <code>"CatchMachineUnderflow"</code> set to <code>False</code>; or better yet just set <code>"RuntimeOptions"</code> to <code>"Speed"</code>.</p> <p>Also, I think the reason you are getting 0 for your <code>AbsoluteTiming</code> command is that you have set the precision too low to represent the result.</p>