Grouping on multiple fields faster when using an array as key?

Working with Data
#1

Hello,

I stumbled into this yesterday and couldn’t find anything about it online. An identical aggregation became much faster than switching the object key to array. Examples:

Slow:

[
  {
    $group: {
      _id: { Field1: "$Field1", Field2: "$Field2" },
      Field1: {
        $first: "$Field1",
      },
      Field2: {
        $first: "$Field2",
      },
      Field3: {
        $push: {
          Value: "$Value",
          Date: "$Date",
        },
      },
    },
  },
]

Fast:

[
  {
    $group: {
      _id: ["$Field1", "$Field2"],
      Field1: {
        $first: "$Field1",
      },
      Field2: {
        $first: "$Field2",
      },
      Field3: {
        $push: {
          Value: "$Value",
          Date: "$Date",
        },
      },
    },
  },
]

Any clues as to why this is? I assume it has something to do with how MongoDB treats the keys internally and I also noticed that the execution plan changed (the one with object using slot-based, the other not).

I haven’t found any examples using an array online, but it yields an identical resultset in our case, just much faster.

Thanks!