FWIW, we initially started our roll-out of the Zimbra Web Client with all our users having IE6. But due to performance and other issues we quickly found that this was not going to fly and upgraded everyone to IE7. In our case at least we have found IE7 much better than IE6. But I must say we still get complaints about slowness. Firefox is by far the best choice in my opinion. You get better performance, less issues, plus the built-in, on the fly spell-checker, which is nice.
Nevertheless what you are describing is unusual behavior. You didn't mention what browser you are using, but I know from personal experience that Firefox works a whole lot faster/better than IE 6 when connecting to ANY Zimbra screen whether admin or mail client. I have heard, but not tried it myself, that IE7 is even worse.