eNewsletter taking up too much wordpress resources

I am using the eNewsletter plugin to send email using the backend of wordpress, but I noticed while the email blast is in progress and there after the website becomes to load very slow.

1) How can I improve performance, and keep the eNewsletter from taking up too many server resources?

2) Is it a good idea to send email blasts using WordPress Multisite, considering the fact that if I offer this feature to other network blogs, this will destroy the performance of the entire network?