The query to MongoDB analytics.interations collection is very slow

Question

I have

• Sitecore 8.1 UPD3(CM, CD, Processing distributed environment)
• mogodb 3.0.15 (sharded cluster and replication)

I am currently facing the problem that accessing CDs takes a very long time to query analytics.Interactions.
This query is as follows: (Excerpt from mongod's log)

$query: { 
  $or: [ 
    { ContactId: BinData(3, 5F98E7DE08126C42B1DC9196379EB7B7) }, 
    { ContactId: BinData(3, 3C1A9FBF535C1A4690FE9B86F14B8105) } 

  ], _t: "VisitData" 
}, 
$orderby: { StartDateTime: 1, _id: 1 } }

According to the log, this execution time takes a very long time of 35245 ms.

Even though {ContactId: 1, ContactVisiteIndex: 1} has an index, I can not understand that it takes such a long time.

How can I get the query to complete in a reasonable amount of time?

All of mongod's log:

2019-03-14T18:25:41.358+0900 I COMMAND [conn65760] query analytics.Interactions query: { $query: { $or: [ { ContactId: BinData(3, 5F98E7DE08126C42B1DC9196379EB7B7) }, { ContactId: BinData(3, 3C1A9FBF535C1A4690FE9B86F14B8105) } ], _t: "VisitData" }, $orderby: { StartDateTime: 1, _id: 1 } } planSummary: IXSCAN { StartDateTime: 1.0, _id: 1.0 } cursorid:47073259001 ntoreturn:0 ntoskip:0 nscanned:1944530 nscannedObjects:1944530 keyUpdates:0 writeConflicts:0 numYields:16100 nreturned:101 reslen:11029 locks:{ Global: { acquireCount: { r: 32202 } }, MMAPV1Journal: { acquireCount: { r: 16105 }, acquireWaitCount: { r: 15 }, timeAcquiringMicros: { r: 3696316 } }, Database: { acquireCount: { r: 16101 } }, Collection: { acquireCount: { R: 16101 }, acquireWaitCount: { R: 4 }, timeAcquiringMicros: { R: 86158 } } } 35245ms

---

2019/03/18

I noticed the following in the log:

planSummary: IXSCAN { StartDateTime: 1.0, _id: 1.0 }

What this IXSCAN shows is that you are using the wrong index.
We considered the following steps to get a valid index, but is it correct?

1st db.Interactions.createIndex( { ContactId:1, _t:1 } . {background: true})
2nd db.Interactions.clear()