I am currently working on a personal project to learn more about mongo. On this project I have a collection with a dynamic field called metadata which can be like this:

metadata: {
"stringField": "string value",
"numberField": 10,
"dateField": "2024-01-01T12:00:000Z"
}

This metadata field can have any number of possible fields inside. So I created a facet search index for the metadata field and their subfields. I then created an aggregation pipeline with a search on that index and facets and tested for 100 million records. The performance was really bad. So I added a limit of 5000 records and now I can perform the search and facets in about 3 seconds. It doesnt return all results, only the first 5k, but that is something. I will post the aggregation pipeline bellow, can you guys help me know if I am doing something wrong or if it is possible to optimize this to run fast (like 1 second or bellow) for that amount of records? I am new to all this.

[
  {
    "$search": {
      "compound": {
        "must": [
          {
            "text": {
              "path": {
                "wildcard": "*"
              },
              "query": "US"
            }
          }
        ]
      },
      "index": "facet-search"
    }
  },
  {
    "$limit": 5000
  },
  {
    "$addFields": {
      "class_id": {
        "$toObjectId": "$classification"
      }
    }
  },
  {
    "$lookup": {
      "from": "Classifications",
      "localField": "class_id",
      "foreignField": "_id",
      "as" : "classObj"
    }
  },
  {
    "$addFields": {
      "contentClassName": "$classObj.name"
    }
  },
  {
    "$unwind": "$contentClassName"
  },
  {
    "$project": {
      "_id": 0,
      "class_id": 0,
      "classObj": 0
    }
  },
  {
    "$facet": {
      "docs": [],
      "DocumentTitle": [
        {
          "$group": {
            "_id": "$metadata.DocumentTitle",
            "count": { "$sum": 1 }
          }
        },
    {
      "$addFields": {
        "displayName": "Document Title"
      }
    },
    {
      "$set": {
        "value": "$_id",
        "_id": "$$REMOVE"
      }
    },
        {
          "$sort": {
            "count": -1 
          }
        },
        {
          "$limit": 10
        }
      ],
      "Customer_Name": [
        {
          "$group": {
            "_id": "$metadata.Customer_Name",
            "count": { "$sum": 1 }
          }
        },
    {
      "$addFields": {
        "displayName": "Customer Name"
      }
    },
    {
      "$set": {
        "value": "$_id",
        "_id": "$$REMOVE"
      }
    },
        {
          "$sort": {
            "count": -1 
          }
        },
        {
          "$limit": 10
        }
      ],
      "Branch_Number_String": [
        {
          "$group": {
            "_id": "$metadata.Branch_Number_String",
            "count": { "$sum": 1 }
          }
        },
    {
      "$addFields": {
        "displayName": "Branch Number"
      }
    },
    {
      "$set": {
        "value": "$_id",
        "_id": "$$REMOVE"
      }
    },
        {
          "$sort": {
            "count": -1 
          }
        },
        {
          "$limit": 10
        }
      ],
    "contentClass": [
      {
          "$group": {
            "_id": "$contentClass",
            "count": { "$sum": 1 }
          }
        },
    {
      "$addFields": {
        "displayName": "Content Class"
      }
    },
    {
      "$set": {
        "value": "$_id",
        "_id": "$$REMOVE"
      }
    },
        {
          "$sort": {
            "count": -1 
          }
        },
        {
          "$limit": 10
        }
    ]
    }
  }
]