A range query using an index like find({a:{$lte:"yadda")}} generates tight bounds:

				"indexBounds" : {

					"a" : [

						"[\"\", \"yadda\"]"

					]

By contrast, a range query by the cluster key like find({_id:{$lte:"yadda")}} generates only a single bound:

		"winningPlan" : {

			"stage" : "COLLSCAN",

			"filter" : {

				"_id" : {

					"$lte" : "yadda"

				}

			},

			"direction" : "forward",

			"maxRecord" : "yadda"

		},

This range query can be quite inefficient as it fetches any document of lower data type, like numeric types.