causing all sorts of problems for us. One thing to note: the master does basically the same updates, but through an normal ldap client, rather than through syncrepl -- and it does not experience this slowness.
syncrepl (re)sends the entire entry, so it's a lot more intense than your typical MOD. I seem to be the only person willing to live with this, but it's quite common to use "delta-syncrepl" instead, which only sends the changed attributes. There should be ample documentation out there; read up and come back if you have questions. (I'll probably change off of traditional syncrepl when I move production to 2.4.)
Apart from that, make sure that your slaves are sanely configured in terms of cache, DB_CONFIG, etc. It sounds like you've got your master tuned to your liking, so you could follow lessons learned there.