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    <p>Based on your given code, we can reverse-engineer the structure of $arr2 to (assuming R, G and B are integer from 0 to 255):</p> <pre><code>$arr2 = array( 0 =&gt; array( 0 =&gt; array( "R" =&gt; 128, "G" =&gt; 64, "B" =&gt; 255 ), 1 =&gt; array( ... ) ) ); </code></pre> <p>Given that your <code>$SIZE</code> is set to <code>256</code>, you will have a total of <code>256*256=65536</code> arrays further containing arrays with key-values for <code>R</code>, <code>G</code> and <code>B</code>, resulting in total of <code>256*256*3=196608 integers</code> in 3 levels of hierarchy. No surprise your code is slow!</p> <p>I think the best strategy here is to try to reduce the total number of items in your array. </p> <p>Given that instead of encoding single cells as "R, G, B" triples, you could encode all values in a single integer. Such as instead of:</p> <pre><code>0 =&gt; array( "R" =&gt; $r, "G" =&gt; $g, "B" =&gt; $b ) </code></pre> <p>Given that <code>0&lt;=r,g,b&lt;=255</code>, you could encode <code>$arr2</code> as:</p> <pre><code>0 =&gt; ($r&lt;&lt;16 + $g&lt;&lt;8 + $b); </code></pre> <p>Now of course you need to unpack the color value inside your loop as well. This can be achieved by:</p> <pre><code>$col = $arr2[$y][$x]; $col_b = ($col&amp;255); $col_g = ($col&gt;&gt;8)&amp;255; $col_r = ($col&gt;&gt;16)&amp;255; $r .= $col_r.":"; $g .= $col_g.":"; $b .= $col_b.":"; </code></pre> <p>This modification alone would cut one level of hierarchy from your array completely.</p> <p>While running your original code with $SIZE=256, my average execution speed in my settings was 0.30 secs. <strong>With the given refactoring, I was able to reduce this to 0.10 secs cutting your calculation time to 1/3 of the original.</strong></p> <p>You will still have a lot of work to do if you wish to improve the performance, but I hope this gives you an idea on how you could proceed.</p>
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