Last findings:
I found out now that the delay occurs from the moment I add the line to create new main window for Tk.

#!/usr/bin/perl

use Tk;

my $G_MW=MainWindow->new;
my $File='d:\\shared\\target\\TTT.html';
system("start /b $File");
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So WITHOUT the ...MainWindow... line => NO problems.
WITH the ...MainWindow... line => DELAY.

The kind of file-extention doesn't seem to matter. I have the same behavior for all filetypes (html,htm,txt,log).