Are you connecting to your LDAP server over SSL?
I am experincing the exact same problem (even with the latest 3.7.0 release) and I don't know what the cause of this is. It would take a good 5-10 min before the group information would come back and would seldomly return (before timeout) when actually viewing group members. Which, is a real pain in the event you want to do group sharing. It gets so bad, the Java memory fills up after some time and locks up the server.
Our network admin put wireshark on our domain controller and saw a constant flow of network traffic from/to the Openfire server. I checked my caching settings and everything looked ok. We have many different applications hitting or domain controller without issue.
I was so happy to see the release notes of 3.7.0 indicating fixed issues with LDAPS but alass it did not fix my issue.
Right now, I simply stay out of the Groups/Users web interface and we are looking at Cisco for future considerations. Too bad, Openfire worked flawless on non-LDAPS connections.