We are running an idle replica set that we have just added a new secondary to. Initial synchronization takes a long time and we have narrowed it down to the primary reading the collection file from the filesystem in 4k chunks. Is this by design or are we doing something wrong? We could surely speed this up by using a larger block size. Obviously the 4k chunk size makes sense on a write-busy primary, but we were wondering whether or not there was any autotuning available to make it read the data set more aggressively while idling.