Why is seq(x) so much slower than 1:length(x)?

Question

I recently answered a question pertaining to for loops. Upon testing my code\'s speed, I noticed that the use of seq() as opposed to :

Answer 1

seq is a generic S3 method, so probably some time is lost dispatching. seq.default is almost 100 lines long!

You're probably already aware of seq_along, which calls a .Primitive directly and is bit better than 1:length(x) and the best method I have found for long loops:

f3 <- function(){
      x <- 1:5; y <- numeric(length(x))
      for(i in seq_along(x)) y[i] <- x[i]^2
      y
  }
>  microbenchmark(f1(), f3())
Unit: microseconds
 expr    min     lq median     uq    max neval
 f1() 27.095 27.916 28.327 29.148 89.495   100
 f3() 26.684 27.505 27.916 28.327 36.538   100