I saw multiple posts on MSDN regarding the following message: 

“The replication agent has not logged a progress message in 10 minutes. This might indicate an unresponsive agent or high system activity.” 

More often than not the above message can be ignored safely. The next obvious question is when can this message be ignored. If you find that your replication agent history is not reporting any error messages, then the above message is benign. Typically, this message means the agent is busy doing its work and has no resources to respond to status inquiries. 

Why does this error message get generated? 

This error message gets generated because of the Distribution heartbeat interval property. This property governs how long an agent can run without logging a progress message. If your replication agents are not reporting an error message and you are seeing the above message, then you could change your heartbeat interval to a higher value. One of the option could be that you changed the history logging option for your replication agent so that it doesn’t log any message. 

Query: 

exec sp_changedistributor_property @property = 'heartbeat_interval', @value = <value in minutes>;

Reference: sp_changedistributor_property (Transact-SQL) 

I have read lots of opinions as to why this is a bug and if you subscribe to that school of thought, please create a Connect item at http://connect.microsoft.com/sqlserver.