Improving query performance of with rails and mongo

Assuming that I have two models:

class Radar
  include MongoMapper::Document
  plugin MongoMapper::Plugins::IdentityMap

  key :name,    String, :required => true
  key :slug,    String, :required => true
  many :stores, index: true

  def self.retrieve_radars_and_stores
    radars = Radar.all
    radars.map { |r|
      {
        name:   r.name,
        ip:     r.ip,
        stores: r.serializable_stores
      }
    }
  end

  def serializable_stores
    stores.map { |store|
      {
        name:     store.try(:name),
        location: store.try(:location)
      }
    }
  end
end

class Store
  include MongoMapper::Document

  key :name, String,     :required => true
  key :location, String, :required => true

  ensure_index :name
  ensure_index :location
end

So, I have a controller method that calls Radar.retrieve_radars_and_stores, getting the result and returning as json.

The code works perfectly, however, having more than 20.000 records, it spends about 1 minute to process the method. Commenting out the stores: r.serializable_stores line, the method spends only few seconds.

My question is, how can I improve this query, reducing the time elapsed?

Thanks!