I've heard response times and download times for email and attachments in a hosted environment are quite slow?

With a modern broadband connection, most customers have more than enough connectivity speed to never notice a difference between a messaging solution they manage themselves and one that is hosted. There is more than enough speed in a DSL, Cable or dedicated Internet line. More importantly, if you have any mobile employees, they can access the messaging solution through any broadband connection, regardless of location, and don’t need to struggle with firewalls that can significantly slow access.