Well,
To resume my tests:
provider and consumer in 2.4.24 -> same behaviour
Get rid of session log -> same behaviour
If I removed one of the providers of my consumer, everything went fine.
Even, if I stop the consumer during mass deletes, it sync very fast as
soon as it starts again
That's related to several providers for one consumer and may be something
else I don't see.
The second provider (first rid in my conf) has less that 15 entries and
no updates at all.
I changed the order of the providers in my slapd.conf. There is no
difference.