Simplistically, it is caused by the required backtracking. How many times it gets called is also dependant upon its relative position:

#! perl -sw
use strict;
use re 'eval';

my $s = 'abcdef';
our $n;

$n  = 0; my @groups = $s =~ m[(?{ print ++$n, ' '; })(?=(..))]g; print
+ $/;
$n  = 0;    @groups = $s =~ m[(?=((?{ print ++$n, ' '; })..))]g; print
+ $/;
$n  = 0;    @groups = $s =~ m[(?=(.(?{ print ++$n, ' '; }).))]g; print
+ $/;
$n  = 0;    @groups = $s =~ m[(?=(..(?{ print ++$n, ' '; })))]g; print
+ $/;
$n  = 0;    @groups = $s =~ m[(?=(..))(?{ print ++$n, ' '; })]g; print
+ $/;

__END__
C:\test>junk57
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 6 7 8 9 10 11
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
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