Gonzalo Bissio covers one reason why initializing merge replication might be slow:
Since 1 week (Yes… 1 week) they were waiting for the replication re-initialization to finish since they introduced some changes on the system and they needed to reinitialize the replication again. The rate of the records between the publisher and the subscribers were too low (20 records per second). Since this application is used globally they write records on all of the regions (then the merge replication manage them).
Read on to see what Gonzalo’s company had going on and how they fixed it.