Parsing a large amount of characters into a POSIXct object

When trying to parse a large amount of datetime characters into POSXIct objects, it struck me that strftime and as.POSIXct where actually quite slow. When using the parsing functions from lubridate, these where a lot faster. The following benchmark shows this quite nicely.

We have a character vector xi, which contains about 2.3 million elements. When using strftime, this takes about 105 seconds on my MacBook pro:

> xi[1:10]
 [1] "2015-06-03 16:17:03" "2015-06-03 16:17:36" "2015-06-03 16:18:09"
 [4] "2015-06-03 16:18:48" "2015-06-03 16:19:21" "2015-06-03 16:19:54"
 [7] "2015-06-03 16:20:27" "2015-06-03 16:21:00" "2015-06-03 16:21:32"
[10] "2015-06-03 16:22:05"

> system.time(res <- as.POSIXct(xi))
   user  system elapsed 
104.941   1.054 106.201

If we switch to ymd_hms from lubridate, we get a very large performance increase:

> library(lubridate)
> system.time(res <- ymd_hms(xi))
   user  system elapsed 
  0.741   0.136   0.878

ymd_hms is about a 120 times faster.