Ethernet networks are adaptive and fault tolerant and will continue working despite partial hardware failures or other problems. Known problems which will degrade network performance, include:
Incorrect or damaged cabling
Routing or lookup tables which have adapted to changes and need rebuilding
Failing Ethernet hardware
Cabling should be visually inspected. Category 5 cable is clearly marked, "CAT 5" and is recommended for 10-Base-T and required for 100-Base-T networks. Any cable which is in questionable condition or is not Category 5, should be replaced with pre-assembled, tested cable.
Reset lookup tables in Ethernet hubs or switches by power-cycling (turn off or unplug for ten seconds, preferably when there is little network traffic). Most Ethernet devices are equipped with indicators for active connections, 10 vs. 100 speed connections, traffic, and collisions, which are fairly intuitive.
If you have more than one hub or switch in your network, it may be helpful to diagram the network connections to ensure there is one and only one route from any node to another, and that no route is more than five "hops."
Test your network with a file transfer. Using a stopwatch, time the transfer of a large file to various machines on your network.