R dataframes copying 3.0 vs 3.1

R_df_copy_3.0vs3.1.R

system.time(z <- 1:1e9)

system.time(d <- data.frame(x = 1:1e9))
system.time(d$y <- 1:1e9)
system.time(d$z <- z)

system.time(d$x[1] <- 0L)


R version 3.0.3 (2014-03-06) 

> system.time(z <- 1:1e9)
   user  system elapsed 
  1.097   0.638   1.739 
> 
> system.time(d <- data.frame(x = 1:1e9))
   user  system elapsed 
  2.982   2.539   5.531 
> system.time(d$y <- 1:1e9)
   user  system elapsed 
  3.572   3.101   6.704 
> system.time(d$z <- z)
   user  system elapsed 
  4.371   4.313   8.713 
> 
> system.time(d$x[1] <- 0L)
   user  system elapsed 
  6.831   6.812  13.730


R version 3.1.0 (2014-04-10) 

> system.time(z <- 1:1e9)
   user  system elapsed 
  0.739   0.635   1.376 
> 
> system.time(d <- data.frame(x = 1:1e9))
   user  system elapsed 
  0.735   0.642   1.380 
> system.time(d$y <- 1:1e9)
   user  system elapsed 
  0.740   0.628   1.371 
> system.time(d$z <- z)
   user  system elapsed 
  0.000   0.000   0.001 
> 
> system.time(d$x[1] <- 0L)
   user  system elapsed 
  0.614   0.746   1.364