I ran this script on some very heavy operations. This involved copying and cloning 70,000 images into 420,000 copies, 3500 users to a mysql database and watermarking all the images. There also were some minor operations involving converting the old propietory PHP site to Drupal.

The operation takes 9 hours and at no time did the web browser ,firefox 2.0, hang or complain. So the only thing I can say since it works perfectly for me is that "your milage may vary".