We recently made a few changes to the design. We now always hedge, immediately, and set a maxTimeMS on the hedged read (configurable via maxTimeMSForHedgedReads runtime server parameter).

Therefore, the code for staggering support should be removed. That includes the code for the "delay" flag in $readPreference and the code for computing the hedging delay. So the user-facing syntax is now: 

{

    ...,

    $readPreference: {

       mode: "nearest",

       hedge:

        { enabled: true, // default value is true }

     } 

}