increased the Delay much more (now 1.5s).

Lower Delay will rise errors when rendering big images. 
My system Win7 64 (slow HD) reports following error on lower Delay: 
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Reached EOF while decoding PNG
Cannot open input file
Cannot create backend data
Unknown fileformat
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Maybe Delay should be bound to the imagesize. Or is there a better solution to wait for povray to finish the writing?