Performance of string concatenation (VimL)

concattime.vim

let lines = readfile('vim/runtime/doc/eval.txt')
echo len(lines) . " lines"
" ----------------------------

" 1. Simplest string concatenation (.=)
let stime = reltime()
let line = ''
for l in lines
	let line .= l
endfor
echo reltimestr(reltime(stime)) . " (" . strlen(line) . " bytes) " . "let line .= l"
unlet line

" 2. Use List with += and join()
let stime = reltime()
let ll = []
for l in lines
	let ll += [l]
endfor
let line = join(ll, '')
echo reltimestr(reltime(stime)) . " (" . strlen(line) . " bytes) " . "let ll += [l]"
unlet line
unlet ll

" 3. Use List with add() and join()
let stime = reltime()
let ll = []
for l in lines
	call add(ll, l)
endfor
let line = join(ll, '')
echo reltimestr(reltime(stime)) . " (" . strlen(line) . " bytes) " . "call add(ll, l)"
unlet line
unlet ll

" 4. Use List with add() with empty check and join()
let stime = reltime()
let ll = []
for l in lines
	if l != ''
		call add(ll, l)
	endif
endfor
let line = join(ll, '')
echo reltimestr(reltime(stime)) . " (" . strlen(line) . " bytes) " . "call add(ll, l) with if"
unlet line
unlet ll

result.txt

Vim 7.4.316

Linux:
8781 lines
  0.327978 (330261 bytes) let line .= l
  0.018020 (330261 bytes) let ll += [l]
  0.021124 (330261 bytes) call add(ll, l)
  0.028716 (330261 bytes) call add(ll, l) with if

Windows:
8781 lines
  3.298255 (330261 bytes) let line .= l
  0.028814 (330261 bytes) let ll += [l]
  0.127366 (330261 bytes) call add(ll, l)
  0.128387 (330261 bytes) call add(ll, l) with if