If you have already maxed out your memory while idling, no wonder everything is slow. Zimbra is using your hard drive as RAM. I would recommend adding another 1 or 2 gb of memory. I understand it is annoying to order parts in South America, and if your government is anything like ours, it will probably be fairly difficult to get them to sign off on it... However, 2gb of server grade ram should not be that expensive.