The only true way to know if the network is slow is to continually monitor actual traffic and performance and compare the results over long term. MRTG is the best tool for the job and has great user support.
Netmon.org is the home of MRTG for Dummies and Indexmaker for Dummies.
Multi Router Traffic Grapher (MRTG) - This is the most widely used tool anywhere on the Internet to graph network traffic. It is portable to almost any operating system. For instant help on getting MRTG set up on a Microsoft Windows configuration, please see MRTG for Dummies and Indexmaker for Dummies. <freeware>
MRTG Statistics Updater - Tool used to run MRTG configuration script as a Windows NT/2000/XP Service. <freeware>
MRTG Configuration File Database and MIB Archive - Somix offers a great archive of MRTG config files. These can simplify creating the config files for complex monitoring. There is also a modest collection of MIB files from main vendors. <freeware>
Novell Netware Monitoring - Some CPU and NLM Monitoring for Novell NetWare in combination with MRTG. <freeware>
MRTG Extension for Netware - Project: MRTGEXT is by far the most current and comprehensive website to integrate MRTG with a Novell Server. Includes updates to Novell Netware 6 SP2. <freeware>
Windows NT/2000/XP Performance Counters - This is an excellent tutorial on using the Microsoft Windows Performance Counters to create MRTG Graphs. Complete with example and confil file script. <freeware>
SNMP4PC - SNMP4tPC (SNMP for the Public Community) includes a lot of information on monitoring Windows NT and Windows 2000 and Novell Netware. There is a low cost product to help install and configure Windows Performance Counters and set up MRTG. <freeware - $>
GetIF - This powerful program allows you to browse the SNMP OID information of any SNMP enabled device. Windows based GUI.<freeware (direct download)>