First check the network adapter settings. Compare to other computers that have higher speeds.
If possible, plug the "slow" computers into a router port being used by a "fast" computer. And vice versa. See if the problem follows the computer or the port.
Try changing any network adapter "auto negotiate" or "auto" settings to manual settings: e.g., speed/duplex. Or if manually set, try auto.
Try known working ethernet cables from another "fast" system on the two computers. Check the wall jack and patch panel connections and connectors if used.
Verify that the two computers have current drivers for the network adapters. Download and update via the manufacturer's website.