1. Yes, they can get errors if they do not have a trusted location set up. If they are using the Runtime (not full version of Access) then they would need to set up the trusted location like this shown here:
http://accessjunkie.com/faq_33.aspx
2. There will be no Ribbons for users who use the Runtime or an accdr with the full version of Access. You need to provide your own Ribbons in that situation.
As for slowdowns you might have to work through the Access Performance FAQ located here:
http://www.granite.ab.ca/access/performanceldblocking.htm