dl.example.com/file1.zip instead of example.com/sites/default/files/file1.zip
In my opinion if you can create a subdomain dl.example.com then you can also redirect all requests to default/files subfolder using symbolic link in /home/domains/dl.example.com/ linking to /home/domains/example.com/sites/default/files/
depending on your .htaccess or httpd.conf setup.

Please note I don't think this is a Drupal-specific question, I am not an expert on this and you'd better asked on https://serverfault.com/

I think using this method will be faster then creating a custom module to do this for you because the method I explained doesn't even require loading Drupal for the request handling.