MongoDB aggregation query that experiences a significant increase in execution time when adding $count or $group stages

Working with Data
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8

Explain output for 1st Lookup stage:

   "totalDocsExamined" : NumberLong(42526),
    "totalKeysExamined" : NumberLong(42526),
    "collectionScans" : NumberLong(0),
    "indexesUsed" : [
        "company_id_1_ledgerName_id_1_createdAt_1"
    ],
    "nReturned" : NumberLong(42544),
    "executionTimeMillisEstimate" : NumberLong(639)

Explain output for 2nd Lookup stage:

    "totalDocsExamined" : NumberLong(42534),
    "totalKeysExamined" : NumberLong(42534),
    "collectionScans" : NumberLong(0),
    "indexesUsed" : [
        "company_id_1_ledgerName_id_1_name_1"
    ],
    "nReturned" : NumberLong(42544),
    "executionTimeMillisEstimate" : NumberLong(9909)