view problem when using array field items to map the view

[Dan Santner]

we have docs that look like this:

_id:id,
seats:[#,#,#,#,#,#….]

and then we are doing a view that maps for each seat in the array.
function(doc) {
   if (doc.type == 'account') {
       doc.seats.map(function(seat) {
           delete doc.seats
           emit(seat.userID, doc))
       }
   }
}

when we get our seat count up to like 1k elements the view is a disaster to 
rebuild.  It pegs the cpu for like a minute.

so we changed this to just emit the id of the doc rather than the doc

function(doc) {
   if (doc.type == 'account') {
       doc.seats.map(function(seat) {
           delete doc.seats
           emit(seat.userID, doc._id))
       }
   }
}

and it rebuilds super fast again.

Would love to know if anyone has hit this before and can explain why it behaves 
this way???