A corrupt OAK index leads to poor performance of query operations with Adobe Experience Manager 6.x.

The following represents a possible OAK index error (within a MongoMK environment, including Oak 1.0.22):

org.apache.sling.commons.scheduler.impl.QuartzScheduler Exception during job execution of org.apache.jackrabbit.oak.plugins.index.AsyncIndexUpdate@7a7eaf04: The node 5:/oak:index/cqLastModified/:index/2015-10-21T18%3A26%3A50.538%2B02%3A00/jcr:system was already added in revision
r15095099f64-0-3, before
r150b8a483bd-0-3; document:
{_lastRev={r0-0-3=r15095099f64-0-3},
_id=5:/oak:index/cqLastModified/:index/2015-10-21T18%3A26%3A50.538%2B02%3A00/jcr:system,
_modified=1446196560,
_modCount=33,
_commitRoot={r15095099f64-0-3=0},
_children=true,
_deleted={r15095099f64-0-3=false}},
revision order:
3:
r150b86e62b1-0-3:r150b86e62ae-0-0 r150b86e66a2-0-3:r150b86e66a0-0-0 r150b86e7e6a-0-3:r150b86e7e68-0-0 r150b86e95fc-0-3:r150b86e95f8-0-0
r150b86e99f0-0-3:r150b86e99eb-0-0 r150b86eb574-0-3:r150b86eb569-0-0 r150b86eb96b-0-3:r150b86eb964-0-0 r150b86ec14a-0-3:r150b86ec148-0-0
r150b86ed58b-0-3:r150b86ed589-0-0 r150b86edd68-0-3:r150b86edd66-0-0 r150b86ee158-0-3:r150b86ee155-0-0 r150b86ef127-0-3:r150b86ef124-0-0
r150b86ef970-0-3:r150b86ef964-0-0 r150b86f05b0-0-3:r150b86f05ac-0-0 r150b86f2f03-0-3:r150b86f2efc-0-0 r150b86f3733-0-3:r150b86f372d-0-0
....
....
org.apache.jackrabbit.oak.plugins.document.DocumentStoreException: The node 5:/oak:index/cqLastModified/:index/2015-10-21T18%3A26%3A50.538%2B02%3A00/jcr:system was already added in revision
r15095099f64-0-3, before
r150b8a483bd-0-3; document:

Solution: Remove the corrupted OAK index

If the index is really corrupted, the only solution is to remove it in mongodb directly.
You can use oak-mongo.js script to do that. See the following URL for more information: 

 http://jackrabbit.apache.org/oak/docs/command_line.html

As it is an index node, you can simply remove it while Adobe Experience Manager is offline.

Then restart Adobe Experience Manager and re-create your index node, it will reindex.