out_sequential_out_o.log

0.1832562: compiled and opened IO::Handle<out_sequential/out_o.log>(opened, at octet 0)
0.1998176: in a single thread
  fp: shakespeare/1591-the_second_part_of_king_henry_the_sixth.book
3.0877887: 2.8873295 read shakespeare/1591-the_second_part_of_king_henry_the_sixth.book 4560(28002) entries; Σ 4560(28002)
3.1011595:                  read shakespeare/1591-the_second_part_of_king_henry_the_sixth.book 4560(28002) entries; Σ 4560(28002) it took 2.90090776sec
  fp: shakespeare/1591-the_third_part_of_king_henry_the_sixth.book
6.1735347: 3.07037366 read shakespeare/1591-the_third_part_of_king_henry_the_sixth.book 3981(27155) entries; Σ 6320(55157)
6.18766135:                  read shakespeare/1591-the_third_part_of_king_henry_the_sixth.book 3981(27155) entries; Σ 6320(55157) it took 3.08463565sec
  fp: shakespeare/1592-the_first_part_of_henry_the_sixth.book
9.2993940: 3.1100717 read shakespeare/1592-the_first_part_of_henry_the_sixth.book 4375(23914) entries; Σ 7964(79071)
9.316804:                  read shakespeare/1592-the_first_part_of_henry_the_sixth.book 4375(23914) entries; Σ 7964(79071) it took 3.1275845sec
  fp: shakespeare/1593-king_richard_iii.book
13.39975347: 4.08061260 read shakespeare/1593-king_richard_iii.book 4578(32529) entries; Σ 9379(111600)
13.4206797:                  read shakespeare/1593-king_richard_iii.book 4578(32529) entries; Σ 9379(111600) it took 4.1015329sec
  fp: shakespeare/1593-the_comedy_of_errors.book
16.00981524: 2.5877451 read shakespeare/1593-the_comedy_of_errors.book 2833(17145) entries; Σ 10100(128745)
16.0275803:                  read shakespeare/1593-the_comedy_of_errors.book 2833(17145) entries; Σ 10100(128745) it took 2.60544657sec
  fp: shakespeare/1594-the_taming_of_the_shrew.book
19.3077522: 3.27876038 read shakespeare/1594-the_taming_of_the_shrew.book 3620(23244) entries; Σ 11113(151989)
19.3348947:                  read shakespeare/1594-the_taming_of_the_shrew.book 3620(23244) entries; Σ 11113(151989) it took 3.3057301sec
  fp: shakespeare/1594-the_tragedy_of_titus_andronicus.book
22.6907185: 3.3538668 read shakespeare/1594-the_tragedy_of_titus_andronicus.book 3805(22650) entries; Σ 11941(174639)
22.7108966:                  read shakespeare/1594-the_tragedy_of_titus_andronicus.book 3805(22650) entries; Σ 11941(174639) it took 3.3741151sec
22.7127638 Stats generation:
23.3477712 Stats:
23.3940084: done

out_withPromise_out_o.log

0.1848754: compiled and opened IO::Handle<out_withPromise/out_o.log>(opened, at octet 0)
0.2028829: with promises
0.20578486 made promise <shakespeare/1591-the_second_part_of_king_henry_the_sixth.book>
  fp: shakespeare/1591-the_second_part_of_king_henry_the_sixth.book
0.20605498 made promise <shakespeare/1591-the_third_part_of_king_henry_the_sixth.book>
  fp: shakespeare/1591-the_third_part_of_king_henry_the_sixth.book
0.2068458 made promise <shakespeare/1593-king_richard_iii.book>
  fp: shakespeare/1593-king_richard_iii.book
0.2079190 made promise <shakespeare/1594-the_tragedy_of_titus_andronicus.book>
0.2077397 made promise <shakespeare/1594-the_taming_of_the_shrew.book>
  fp: shakespeare/1594-the_tragedy_of_titus_andronicus.book
  fp: shakespeare/1594-the_taming_of_the_shrew.book
0.206261880.20737814 made promise <shakespeare/1593-the_comedy_of_errors.book>
  fp: shakespeare/1593-the_comedy_of_errors.book
 made promise <shakespeare/1592-the_first_part_of_henry_the_sixth.book>
  fp: shakespeare/1592-the_first_part_of_henry_the_sixth.book
20.71412091: held promise and read shakespeare/1591-the_second_part_of_king_henry_the_sixth.book 4560(28002) entries; Σ 4560(28002) it took 20.5274801sec
21.72606213: held promise and read shakespeare/1591-the_third_part_of_king_henry_the_sixth.book 3981(27155) entries; Σ 6320(55157) it took 21.5241056sec
23.15874457: held promise and read shakespeare/1592-the_first_part_of_henry_the_sixth.book 4375(23914) entries; Σ 7964(79071) it took 22.9713144sec
24.79771647: held promise and read shakespeare/1593-king_richard_iii.book 4578(32529) entries; Σ 9379(111600) it took 24.6171600sec
26.3205637: held promise and read shakespeare/1593-the_comedy_of_errors.book 2833(17145) entries; Σ 10100(128745) it took 26.12022201sec
28.0417359: held promise and read shakespeare/1594-the_taming_of_the_shrew.book 3620(23244) entries; Σ 11113(151989) it took 27.84465189sec
29.92410491: held promise and read shakespeare/1594-the_tragedy_of_titus_andronicus.book 3805(22650) entries; Σ 11941(174639) it took 29.7285703sec
29.9535147 Stats generation:
30.85693410 Stats:
30.91356470: done

Word_stats.pl6

#!/usr/bin/env perl6
# Die Okt 27 12:58:06 CET 2015
###
use v6;

my $tBegin  = BEGIN { now };
my $tInit   = INIT { now };

# SUBS
###
#  bag-file
#  ->  file name, log file handle
#  <-  bag of words found
sub bag-file( $sFilename, $hFo ){
    $hFo.say: "  fp: $sFilename";
    return ($sFilename, now, $sFilename.IO.slurp.comb(/\w+/).Bag);
}

###
#  MAIN
#  ->  $outdir stats written to outdir
#      $withPromise 
#      @asInfiles set of files to be processed

sub MAIN( :$outdir='out', Bool :$withPromise=False, *@asInfiles){
    my $fo-log = "$outdir/out_o.log".IO.open(:w);
    $fo-log.say: $tInit - $tBegin, ": compiled and opened ", $fo-log;
    my $bagDb = ().BagHash;

    if $withPromise {
        $fo-log.say: now - $tBegin, ": with promises";
        my @promises =     @asInfiles.map( -> $sFile {
            Promise.start({
                $fo-log.say: now - $tBegin, " made promise <$sFile>";
                bag-file( $sFile, $fo-log );
            })
        });
        
        for await @promises  -> ($sFile, $starttime, $bag0) {
            $bagDb ⊎= $bag0; 
            $fo-log.say: now - $tBegin, ": held promise and read $sFile {$bag0.elems}({$bag0.total}) entries; Σ {$bagDb.elems}({$bagDb.total}) it took ", now - $starttime, "sec";
        }
    } else {
        $fo-log.say: now - $tBegin, ": in a single thread";
        @asInfiles.map( -> $sFile {
            my ($sFilename, $starttime, $bag0) = bag-file($sFile, $fo-log);
            $bagDb ⊎= $bag0; 
            $fo-log.say: now - $tBegin, ": ", now - $starttime, " read $sFilename {$bag0.elems}({$bag0.total}) entries; Σ {$bagDb.elems}({$bagDb.total})";
            $fo-log.say: now - $tBegin, ":                  read $sFile {$bag0.elems}({$bag0.total}) entries; Σ {$bagDb.elems}({$bagDb.total}) it took ", now - $starttime, "sec";
        });
    }

    $fo-log.say: now - $tBegin, " Stats generation:";    
    my %hStat;

    for |$bagDb.kv -> $k, $v {
        push %hStat{$v}, $k;
    }
    $fo-log.say: now - $tBegin, " Stats:";    
    my $fo = "$outdir/out.o.stat".IO.open(:w);
    $fo.print("$_\n") for %hStat.pairs.sort(+*.key).reverse;
    $fo-log.say: now -$tBegin, ": done\n";
}
#   vim:        expandtab sw=4 ts=8 sts=4 ai 

We have shakespeare's complete publications as simple txt source gutenberg project in a subfolder.
The script collects the words in bags and prints the statistics to an outfile "$outdir/out.o.stat" and logs to "$outdir/out_o.log".
The parsing is done sequentially or --withPromise uses promises.

./Word_stats.pl6 --outdir=out_sequential shakespeare/159[1234]_book
./Word_stats.pl6 --withPromise --outdir=out_withPromiseshakespeare/159[1234]_boo

But somehow --withPromise is slower. Am I doing some thing wrong?