Richard MacManus on ZDNet reports about ZDNet readers commenting on his first review of the Zimbra Office Suite saying the suite is way too heavy for older hardware. Especially the fact that a comparison between GMail and Zimbra on older hardware reveals this very clearly is somewhat worrying :

"the GUI is way too slow: it takes over 4 minutes to load on my old Pentium III, while GMail loads in < 10 seconds."

According to Zimbra issues exist within Internet Explorer :

because IE lacks decent DOM manipulation techniques.

Link : Zimbra.com
Review of Zimbra on this blog : Office Revolution