in reply to Threaded Code Not Faster Than Non-Threaded -- Why?
Also bear in mind that since this appears to be “calculating file digests,” this is necessarily an I/O-intensive task, not a CPU-intensive one: the content of the file must be retrieved from disk in order to be digested. The ruling constraint will be the capacity of the disk-drive to seek ... to move the read/write head from one track to another. There will be some opportunity for threading here, but perhaps not nearly so much as you might initially have supposed.
In Section Seekers of Perl Wisdom