Rendering is too slow for file containing many shades

Attached pdf files take too long (more then 20 secs) to render in PDFDebugger (they are a simplified version of a real life example that I can not publish for privacy reasons).

Displaying more shades seems to increase complexity exponentially: all the attached files contain the same shades but each one display a different range of them.

These are the rendering times as displayed in the status area of PDFDebugger for each of the attached files:

0to25.pdf: 22.095 seconds
25to50.pdf: 33.726 seconds
0to50.pdf: 67.481 seconds