Mmmm, if I read correctly, a single lookup takes, on average, 6 secs - if it takes 6 secs/entry, it should take about 330,000 secs i.e. a lot longer than 18 hrs, for 55,000 entries, so I think you're getting off lightly!

It strikes me that the best answer to your problem is, as jbert suggests, to find a bulk LDAP download tool and use that - which will inevitably require knowledge of LDIF file processing.