When a large array has format

a: [{b: 1}, {b: 2}, ...]

and aggregation does something like

{$slice: ["$a.b", 1, 2]}

then as array "a" gets larger, materializing it into "$a.b" becomes more expensive - and just taking $slice (or $arrayElemAt) and then extracting "a.b" can be significantly faster.

Discovered when working with very large arrays (over 10,000) and doing $slice and two $arrayElemAt operations in the same stage and having it perform 100x worse than explicitly taking slice first and then extracting b out of smaller result.