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.