It turns out this defect has nothing to do with file I/O at all.

The following code can reproduce the effect. I have raised a bug ticket with perlbug and the compiler of perl for cygwin.

## Test code to show split slow when used in muiltiple threads is slow
+ in cygwin perl
use strict;
use warnings;
use threads;
use Thread::Queue;
use Time::HiRes();

my $string = "12356789 "x25;
my $TASK_COUNT =10;

print( (`$^X -v`)[1]);
thread_process(1);
thread_process(4);

sub thread_process {
    my $parallel = shift;
    my $start = Time::HiRes::time();
    my @threads;
    my $queue = Thread::Queue->new();
    for my $p ( 1 .. $parallel ) { 
        push @threads, threads->create( sub {
            while( my $task_no = $queue->dequeue) { 
                for my $n (1..10_000) {
                    my @x= split / /, $string;  # Split 
                }
        }})
    }
    $queue->enqueue( 1 .. $TASK_COUNT  , (undef) x $parallel );
    $_->join for @threads;
    my $end= Time::HiRes::time();
    printf "Processing %d tasks in %d threads completed in %fsecs\n", 
+$TASK_COUNT, $parallel, $end-$start;
}
__END__ 

# Here is an example of the output on my dual core XP machine

$ ~/localperl/bin/perl.exe example.split.pl 
This is perl 5, version 14, subversion 1 (v5.14.1) built for cygwin-th
+read-multi-64int
Processing 10 tasks in 1 threads completed in 2.640625secs
Processing 10 tasks in 4 threads completed in 12.468637secs

$ perl example.split.pl 
This is perl, v5.10.0 built for cygwin-thread-multi-64int
Processing 10 tasks in 1 threads completed in 4.578125secs
Processing 10 tasks in 4 threads completed in 25.437345secs

# The defect does not occur with ActivePerl. Instead the expected slig
+htly faster result occurs
$ c:/perl/bin/perl example.split.pl 
This is perl 5, version 12, subversion 3 (v5.12.3) built for MSWin32-x
+86-multi-thread
Processing 10 tasks in 1 threads completed in 2.062500secs
Processing 10 tasks in 4 threads completed in 1.671790secs
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