by Iñaki Arenaza.
Is you LDAP server in a completely different network location than your Moodle server?
If this is so, network latency could make a huge impact in synchronization times. For the sake of the argument, assuming a 1 second round-trip latency for user update query (and you need at least one LDAP query per user update), that means your 17K spend 4.7 hours just in network latency (without taking into account any processing time either in the LDAP server or the Moodle server).
On the other hand, having your two servers connected to the same network gear (switch/router/whatever) can bring that latency down to a few dozens of microseconds (again just talking about network latency, I'm not counting processing time that should be the same in both scenarios).
You could add some instrumentation code (using PHP's microtime() function) to measure the time it takes to perform the relevant LDAP queries (just the queries, to minimize PHP execution times, etc.). And then measure them in both your private server and the MoodleRooms server and see if it makes any difference.
Saludos. Iñaki.