Yes, the model is essentially one of a central write-master with all clients being replicas (using our own replication technology). We do want to think about moving away from this, however, possibly using local proxy-caching with a handful of servers replicating from the master with syncrepl. It's all still to be investigated properly, but there's certainly an incentive for that.
Ok, well you should have really mentioned "using our own replication technology", and since we have no way of knowing what this is or why you are using it, we can't possibly help diagnose if this is having an effect on slapd, other than what we have already discussed.
Why not use an open/rfc'd proven replication technology?
Again, decisions from the past. Yes, apologies, I should have mentioned our own replication, although I don't believe this to be much of a factor here - in essence the replication is a pull-based form of synchronisation which knows the last modification date of the local directory and then queries the master for entries modified after that date. It uses standard LDAP operations to make the changes to the local directory.
Cheers Toby