You should be able to sustain Gbit speeds PC-to-PC providing your router is the rev that provides Gbit LAN ports, your PC's are equipped w/ Gbit ethernet cards, and the ethernet wiring is under the limiting distance for transferring at Gbit speeds, ie cat5e (roughly under 100' max), cat6 (roughly under 300' max) and of course all twisted pairs and ethernet terminations (jacks/crimps) are also of good quality.
Rare but...other factors that can play a role is how the cabling is ran as I've seen where other nearby electrical wiring or equipment can cause issues due to electric noise. Another easy thing to swap out is the patch cords as I've sometimes found they can be the source of a poor connection. Lastly, check the pins in the wall outlets and make sure there is no green/white corrosion on the copper too, if so replace the jacks and look into why there is corrosion in the first place.
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