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

A more specific reason why it's slower:

seq(x) will call seq.default*, and seq.default calls 1L:x!!

From seq.default:

if ((One <- nargs() == 1L) && !missing(from)) {
    lf <- length(from)
    return(if (mode(from) == "numeric" && lf == 1L) {
        #checks validity -- more slow-down
        if (!is.finite(from)) stop("'from' cannot be NA, NaN or infinite")
        #boom! under the hood, seq.default is doing 1:N
        1L:from
    #looks like it defaults to seq_along if length(from) > 1?
    } else if (lf) 1L:lf else integer())
}

*Unless, of course, x is Date or POSIXt, or you have another library loaded that has a seq method...