I'm logging sync logs, but I haven't found much information about these logs. Is there any way to debug it? How?
Unfortunately, you've not really provided any actionable information, so it's hard to say. I can say that I've had MMR deployments for years, using delta-syncrepl without delay issues like you are reporting. There are several examples of setting up N-way delta-syncrepl MMR servers in the tests/data/regressions area, for example the regression tests for ITS8444 and ITS8800.
Regards, Quanah -- Quanah Gibson-Mount Product Architect Symas Corporation Packaged, certified, and supported LDAP solutions powered by OpenLDAP: <http://www.symas.com>