[cairo] will clip region decrease the performance of filliing?

[oken]

Hi,all,
  In what situation will the clip region decrease the performance of filliing?
  I find it that I my program if I reset the clip before filling, it will be much faster.
  It my program, if i don't reset the clip, calling cairo_fill to fill 500 rectangles takes about 20ms, which i think is too long!
  Any suggestions to improve the performance of cairo_filling?

Thanks.
  
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