We're in the process of migrating from a simple Linux/Postfix/POP3 email environment over to Zimbra (OSS), with the clients switching from Outlook to the Zimbra Web Client. We are approximately 1/3rd of the way done, with about 125 current Zimbra users. The users are spread across four locations, with the three remote locations being connected via fractional T1 circuits. The biggest complaints so far from the users have centered around slow response of the web client; from it takes longer to add/open attachments, to slowness when moving between folders, to some users getting "The server appears to be slow to respond" error message. I think some of this is just the users getting comfortable with a new program, and some it can be explained away with the trade-offs of the added features of Zimbra versus a little bit slower response time.

I think (hope) our server is adequately sized. We have dual Xeon 3.4 GHZ processors, with 2 GB of memory, and approximately 400 GB of disk space spread across six spindles (RAID 1+0). Monitoring the server with 'top' typically shows the CPU 90+% idle, with very little or no wait time. The two biggest memory hogs according to top are java, using typically 25% of the memory, and mysqld using about 11-12%. These numbers haven't moved much as we've added new users. I know our WAN is not the speediest, and this will obviously impact Web Client performance, but some of the slowness complaints have come from LAN users as well. Also, I haven't noticed any spikes in WAN traffic; ping times between the local network and the remotes has remained pretty much the same as they were before we started using Zimbra.

So, having given all that background information, I'm looking for ways to quantitatively troubleshoot these performance issues so that we can put our finger on specific issues that need to be addressed.

Also, I would appreciate any tips for improving Web Client performance. Two areas I am currently looking at:
1. The browser being used - when we started this project pretty much all our users were using IE6. As these performance complaints have come in we've been upgrading to IE7 and this definitely seems to help. I'd love to move to Firefox, but our organization is, unfortunately (in my opinion at least) still very much Microsoft biased, so unless I can show day/night performance difference I doubt this will happen. What's the general consensus about IE7 performance compared to FF?

2. Secure (https) connections versus regular http - One of the big selling points of Zimbra was access to email, calendar, contacts, etc. anywhere in the LAN/WAN/Internet. So keeping in mind that people would be connecting over the Internet I configured Zimbra for https. I'm sure this could have negative impact on performance, but what are my other options? Is there any way, even if 3rd party software is necessary, to make connections via the Internet use https, but allow internal (LAN/WAN) connections through regular http?

I've been one of the main drivers of the move to Zimbra, and I really want to see our project succeed, so any thoughts, comments, suggestions, idea, etc. are welcome and would be very much appreciated.