build.sbt

scalaVersion := "2.10.3"

resolvers ++= Seq(
  "Scalaz Bintray Repo" at "http://dl.bintray.com/scalaz/releases",
  "Sonatype OSS Snapshots"  at "http://oss.sonatype.org/content/repositories/snapshots/",
  "Typesafe repository" at "http://repo.typesafe.com/typesafe/releases/"
)

libraryDependencies ++= Seq(
  "com.typesafe.play" %% "play-json" % "2.2.2-RC1",
  "org.scalaz.stream" %% "scalaz-stream" % "0.3.1",
  "org.scalaz" %% "scalaz-concurrent" % "7.0.6",
  "org.typelevel" %% "shapeless-scalaz" % "0.2-SNAPSHOT"
)

PlayJsonDemo.scala

import scalaz._, Scalaz._, concurrent.Task, stream.io.linesR
import shapeless._, contrib.scalaz._
import play.api.libs.json._, play.api.libs.functional.syntax._

case class Result(deletions: Int, entities: Int, colors: Map[String, Int])

object Result {
  val deletion = Result(1, 0, Map.empty)
  def tweet(entities: Int, color: String) =
    Result(0, entities, (color != "C0DEED") ?? Map(color -> 1))
}

object PlayJsonDemo {
  val entitySum = (
    (__ \ 'hashtags     ).json.pick[JsArray].map(_.value.size) and
    (__ \ 'symbols      ).json.pick[JsArray].map(_.value.size) and
    (__ \ 'urls         ).json.pick[JsArray].map(_.value.size) and
    (__ \ 'user_mentions).json.pick[JsArray].map(_.value.size)
  )((a, b, c, d) => a + b + c + d)

  val resultReads = (__ \ 'delete).json.pick.map(_ => Result.deletion) or (
    (__ \ 'entities).read(entitySum) and
    (__ \ 'user \ 'profile_background_color).read[String]
  )(Result.tweet _)

  def result: Task[Result] = linesR(
    "../sample-all-2014-03-16.json"
  ).gatherMap(16)(
    line => Task(Json.parse(line).as(resultReads))
  ).runFoldMap(identity)
}

In response to a challenge by Christopher Brown. On just over a million tweets on my dual-core laptop, the Play implementation takes about 39 seconds, Json4s about 38, and Argonaut about 51.

It's cool to see that with functional composers from Play-Json, we don't lose much time compared to more "imperative" code like json4s (which you could also write for play-json)...
I wonder how it behaves with JsZipper ;)