R fastest way to convert an entire data table to lower case

Question

I have a function written as:

setlower <- function(df) {
  for(j in seq_along(df)){
    data.table::set(df, j=j, value=stringi::stri_trans_tolower(df[[j]]))
  }
  invisible(df)
}

I have a much larger package that calls this function on multiple data.table's in an lapply. The profile for it looks like:

You can see I spent a majority of my time for each data.table I am processing in the setlower function. Is there anyway to speed this up? I know both data.table and stringi are supposed to be about as fast as you can get for data frame and string operations but I didn't know if there is a potentially faster way to assure that any given data frame is converted entirely to lower case.