Computers on your network can operate slowly for many reasons. This list is to help troubleshoot and give suggestions on what to check.

• Cat5 cabling (networks may still operate with not all wires working in a network cable)
• DNS misconfigation (switching internet providers can uncover machines with hard coded DNS entries. If a sration has one bad DNS and one good will create slowness while the first DNS times out each time it turn comes up)
• switch and network card not handshaking on 10/100/1000 (a switch thinking it is speaking to a 100mb network card when the network card thinks it is talking to a 1000mb switch will still work but with mass packet failure) network card settings in windows and managed switches can be hard set, if you choose to hard set a swich stick some tape on the switch to remind you or future techs when the workstation gets swapped later on.
• packet storms create by misconfigured router, VPN tunnel or workstation
• malware or virus installed on a workstation causing network traffic
• old software driver or firmware on router (check for updates for your software and hardware)
• bad power, get a UPS (Computers as they have gotten more efficient now operate on lower and lower tolerance on voltage, a machine that worked fine may not when upgraded)
• even with a UPS really noisy power can cause network slowness (welders, air conditioners, compressors and other large equipment can create micro brownouts entry level UPS devices can't clean. Typically this is a speradic network failure when the machine(s) operate.