Very slow merge replication synchronization
There are many factors that can affect replication. Some common factors that I've come across are related to slow networking, blocking and storage issues. But the one that may be a factor is the performance problem with virtual log files. If you are initializing a database with a large amount of data and the default growth factors are in place, then sql server will grow the data 1mb at a time and the log 10 percent at a time. To check for the VLF issue, run dbcc loginfo. If that command returns over 100 records, I would be concerned. If it returns thousands of records, then it will have a real impact on performance. There are plenty of articles written on this subject. The basic fix is to adjust the autogrowth settings on all data and log files to reasonable sizes, then shrink the log file and initialize it back to the original size. I would check all of the databases, including the distribution database. There could also be other reasons why the distribution database is slow to distribute the transactions. I've experienced this on several transactional replication systems in the past and was suprised at the impact of correcting this every time. 

I would also suspect locking at the publisher. Identify the spids involved in the replication and use the dynamic management view sys.dm_os_waiting_tasks to determine which waits are involved in these sessions. This will help identify what these subscriptions are waiting on.