Slow work after block port 80
I am use Zimbra OSE 8.0.2.
On this server I've set up "https only" access mode. In our company we have 2 groups of users: first group with internet access; second - without internet, but with access to internal corporate resourses. Firewall rules for second group of users include block any activity to port 80 (http protocol). As I say, zimbra server works on https (443 port), but we have problem with users from second group (80 block, but 443 on zimbra server accept). This problem is slow work (zimbra authorization page and sending emails from web-interface) in comparison with users at which the port 80 is open. Why blocking of port 80 creates problems if the server works at port 443?