As soon as I read that your browser locks up, thr first thought that came to my mind is that you could create a page that when you hit submit, it takes you to a page that has some kind of message that says "Sending.. please wait." or something, then at page will actually call the action page that processed what you needed it to do. This won't fix the lock up but it will tell the users that it is working. Or you could have an ajax page pop up saying it's working.

What are the emails sent out for? Because a CRON job would take time to do and when it ran, wouldn't it most likely slow up the entire server as well?