Mongodb replica config slaveDelay but data still sync immediately
If secondary do "initial sync", it will reach primary first, but then it will "sleep" (this case 600 seconds) to fall back and after that, it will keep syncing 600s behind.

You can see that (sleep) in mongod.log if you watch what is happening at end of initial sync.  After that sleep, if you look "last" document in opLog, it's time stamp is that 600s behind. Of course you can see that with rs.printSlaveReplicationInfo() too.