Immediately after migrating a mongodb replica set from 2.4.5 to 2.6.9 we observed an increased lock % on all members, even on secondaries that were only synchronizing without serving queries.
Our replica set is composed by 4 nodes -> 1 primary, 2 slaves and an hidden one. We do not use sharding. We rolled back all nodes except a slave in order to troubleshoot and diagnose the issue.
We attached iostats output and mongo mms (please consider interval 09-11 AM) graphs collected at the same time on the two slave nodes with different mongo versions.
As you can see the node with mongo 2.6 shows increased lock % and background flush avg. Also the amount of data written to the recovery log and database datafiles per second are much more bigger on mongo 2.6.