Codez un logiciel

I have a Drupal website with simplenews module. If I set the Cron Throttle to 500 it works. When I set it to 1000 it's not working and I get no error message. I notice by checking the server process list that when I run cron with for example 500 emails. Cron takes about 1hour and a half to complete. The problem is that I cron takes a long long time and sending emails is done in a few minutes. So I dont know why but cron takes long time to complete howherver sending mails took only a few minutes. But as the cron take a lon time, the all process is slow.