"The folder and subfolders contain several million files with thousands of folders."

Ouch.... Since DIRPlus has to crawl and look at each file, building an array as it goes... yeah it could take some time.

You might want to try a low tech method....

 Code:
shell '%comspec% /c "f:\reports\trans\monthold" /s /a-d | find /i "' + $reporttype + '" > Files.txt'


If this directory structure is as big as you say it is, your files.txt file could be several megs large by time it is finished....