Some find operations on a mongodb sharded cluster we have are very slow, and when running a db.collection.explain() to figure out how which is the execution plan is also taking a very long time to execute (more than 30 minutes).

The command we are issuing to obtain the explain plan is the following:

db.ocs.explain().find({ "site": 1, $and: [ { "TID": { $gte: 195411511, $lte: 198134790 } }, { $and: [ { "adrTyp": "topup" }, { $and: [ { "strDate": { $gte: ISODate("2017-10-24T15:00:00.000+0000"), $lte: ISODate("2017-10-24T16:00:00.000+0000") } }, { $and: [ { "ocs.res.ocs": { $gte: 12, $lte: 14 } }, { $and: [ { "chn": "MULA" } ] } ] } ] } ] } ] })

Is this a normal behavior? how can we understand why the explain command is taking so long?