Unless additional details can be provided (for example, screen shots of error messages or informative messages), it is recommended that you restart your server and workstations.
How often do you restart your server(s)? Restarting Windows-based servers at least once a month, whether it appears necessary or not, is a widely accepted industry standard. The Windows operating system is not known for its exemplary housekeeping abilities (allowing buildup of residual processes, maintaining locked records and files unnecessarily, and other anomalies which cause problems of this nature).
When did the symptoms first start occurring? Establishing a timeline can help locate potential problems while reviewing Event Viewers and other network related log files on servers and workstations as error logs are date and time stamped. Packet Collisions, Broadcast Storms, even multiple users streaming Internet can cause serious performance problems on a network. IT department personnel should monitor these issues as a matter of procedure or preventative maintenance.
What has changed on the network? Examples might be new software installations and/or upgrades? Have there been any new hardware installations or upgrades? Have there been any problems with power in the building?
Any one, or a combination, of these items can contribute to network latency issues.