我有一个非常简单的集合，除了_id之外没有索引。每个文档都包含一个数组字段。当前的集合大小大约为1亿，我可以在探查器中看到以下异常:

{
  "op": "update",
  "ns": "DB.links",
  "command": {
    "q": {
      "_id": "f1b54f37-7f92-4e75-9ec6-5329349ce792_eb370c8a-6f33-4989-aa59-a26e1c9df46c"
    },
    "u": {
      "$push": {
        "mies": {
          "$each": [
            {
              "$oid": "5e39d07bec34b8000e7f86b7"
            }
          ]
        }
      }
    },
    "multi": true,
    "upsert": true
  },
  "keysExamined": 0,
  "docsExamined": 0,
  "nMatched": 0,
  "nModified": 0,
  "upsert": true,
  "keysInserted": 1,
  "numYield": 0,
  "locks": {
    "Global": {
      "acquireCount": {
        "r": 2,
        "w": 2
      }
    },
    "Database": {
      "acquireCount": {
        "w": 2
      },
      "acquireWaitCount": {
        "w": 1
      },
      "timeAcquiringMicros": {
        "w": 19486143
      }
    },
    "Collection": {
      "acquireCount": {
        "w": 1
      }
    },
    "oplog": {
      "acquireCount": {
        "w": 1
      }
    }
  },
  "millis": 19490,
  "planSummary": "IDHACK",
  "execStats": {
    "stage": "UPDATE",
    "nReturned": 0,
    "executionTimeMillisEstimate": 0,
    "works": 2,
    "advanced": 0,
    "needTime": 1,
    "needYield": 0,
    "saveState": 0,
    "restoreState": 0,
    "isEOF": 1,
    "invalidates": 0,
    "nMatched": 0,
    "nWouldModify": 0,
    "nInvalidateSkips": 0,
    "wouldInsert": true,
    "fastmodinsert": false,
    "inputStage": {
      "stage": "IDHACK",
      "nReturned": 0,
      "executionTimeMillisEstimate": 0,
      "works": 1,
      "advanced": 0,
      "needTime": 0,
      "needYield": 0,
      "saveState": 0,
      "restoreState": 0,
      "isEOF": 1,
      "invalidates": 0,
      "keysExamined": 0,
      "docsExamined": 0
    }
  }
}

正如你所看到的，一个简单的upsert单推到阵用了19秒。我相信大部分时间都是在这里度过的:

      "timeAcquiringMicros": {
        "w": 19486143
      }

我应该检查什么？如何提高性能？