Exchange 2007 was the last version of Exchange to have single instance storage. Therefore Exchange hasn't got to make unique copies of each message.
Do you see the message in the queues during this delay?
Although I would be discouraging people to send a 10mb attachment to such a large number of users. There are a lot more efficient ways of sharing the data.
How large an attachment? The amount of time seems extraordinary, but most people don't realize that when they send a large attachment internally to a large group of people, especially if it's REALLY large (like over 50MB or so), it can heavily impact Exchange performance for older versions (and I'm not really sure if this has been fixed with Exchange 2013/2016). Exchange has to create a copy of the attachment for each member of the distribution list and then store it in that user's mailbox. Then it may possibly have to be downloaded to the user's workstation if they are working in cached mode. If that's what's going on, it's a training issue, IMO. Especially if it's an internally-generated document/video/image file or whatever, users should be trained to save it to a common location where everyone can access it and then send a hyperlink to it rather than a copy to each user.
Do you see the message in the queues during this delay?
Although I would be discouraging people to send a 10mb attachment to such a large number of users. There are a lot more efficient ways of sharing the data.