Exchange Server 5.5 slow response

I have a Exchange Server 5.5 installed in my company, with the NT40 domain providing services to almost 70 users. At the Client_End, win98, I am using outlook 2000, but some time some clients complaints that outlook is not responsing to finding Exchange Server. Please guide how can I rectify this problem, either I should devide my network with 2 subnet with 2 cards or any other solutions to increase the performance of my Exchange server.. please guide in detail..
Regards… Aamir …