Individual bulk e-mails - performance question

Hi everybody, I am running a travel community where users want to get informed on changes inside different groups they have subscribed to. At the moment I am doing this with a for loop that generates an individual e-mail sent to them via phpmailer. That works, however the submit of the content upload form (that triggers the e-mail notification) now takes several seconds, as more and more users subscribe. I am thinking about placing the info on the individual e-mail inside a ascii txt file that will be read by a cron job which will send the e-mail instead. Something like every 5 minutes reading it line by line and then after sending it removing the line. e.g: for:bla@blub.de; body:individual for:new@something.com; body:other email Does this sound like a plan? Or do you believe that there are better ways doing it? I could imagine that I would run into problems a few months from now if for example the cron job will be triggered a second time, while the first one has not finished. Any ideas or suggestions? Thank you for any help. Best regards, Merlin