I was looking at the optimizing aggregation pipelines docs page and didn't see this one listed.

I don't know what the cost is to do a bunch of projections only to take the first 3 of them, but reordering the pipeline to push $project before limits and skips would remove the question.

So:

{ $project: { a: 1, b: 1} },
{ $skip: 10 }

would look like this:

{ $skip: 10 },
{ $project: {a :1, b: 1 } }

In addition, doing this might allow further skip/limit coalescence to further optimize the pipeline.