optimizing arrays

Hi There, What techniques are there to optimise the use of very large arrays? I have an array of over 380K keys and values that is being processed incrementally, however the longer it runs the more time is seems to be taking - an array of 100k items is taking 1 hour to process, and array of 400k items is taking 9 hours to process. I'm chunking up the array into smaller arrays, but that doesn't seem to be making much difference. Thanks Anna