Batch Processing is a method of processing multiple orders in bulk, when this is done, the order status is flagged to update (on the channel on next sync), emails are sent (if set) and orders are marked as processed.
Sending E-Mails on Process
When you process orders there is an option to send notification emails to your customers, if you are batch processing orders then this can slow down the process as multiple emails need to be sent at a time.
The disadvantage of not sending emails however is that your outbox for emails will get full quite quickly and emails will need to be sent manually.
Incorrect SMTP Details
If you are attempting batch process orders and your smtp details are incorrect then you will have lots of failed emails, this will cause delays when processing orders as the failed emails need to be logged.