test<-function (sleepstudy){
  print(nrow(sleepstudy))
  print(system.time(
    ddply(sleepstudy, .(Subject, Days), summarise,
          avg_rt = mean(sleepstudy[sleepstudy$Subject != Subject &
            sleepstudy$Days == Days,"Reaction"]))
  ))

  print(system.time(
    do.call("rbind", (lapply(unique(sleepstudy$Subject),
                             function(x)
                               cbind(Subject = x,
                                     aggregate(Reaction ~ Days,
                                               subset(sleepstudy, Subject != x),
                                               mean)))))
  ))
}

sleepstudy<-lme4::sleepstudy

for (i in 1:5) {
  sleepstudy<-rbind(sleepstudy,sleepstudy,sleepstudy,sleepstudy)
  test(sleepstudy)
}