In SERVER-87589, we limited the number of rewritten $first/$last accumulators only to 2 as overhead of generating the same sort key multiple times for multiple rewritten $top/$bottom could be big if there are too many $first/$last.

Suggested idea:

{$sort: {sortField: 1}},  
{$group: {_id: ..., firstX: {$first: "$x"}, firstY: {$first: "$y"},  
                              lastX: {$last: "$x"}, lastY: {$last: "$y"}}}  

--> becomes -->

{$group: {_id: ...,
  tops: {$top: {sortBy: {sortField:1}}, outputs: {x: "$x", y: "$y"}},  
  bottoms: {$bottom: {sortyBy: {sortField:1}}, outputs: {x: "$x", y:"$y"}}},
{$project: {
  firstX: "$tops.x", firstY: "$tops.y", lastX: "$bottoms.x", lastY: "$bottoms.y"}}  

Update #1:
anna.wawrzyniak@mongodb.com suggested to implement a generalized optimization like this where, if there are several topNs/bottomNs on the same patterns, we use a single sorter instead of having a different sorter for each accumulator.

Update #2:
As part of this improvement, we should be able to increase internalQueryMaxFirstLastRewrites's default value to a much higher one.