This issue can have many root causes. Please attempt the following, in this order, checking after each step if the issue persists:

  1. On the specific PC's check that NIC is not set to auto negotiate for the connection rate. It might be defaulting to 10/100 instead of 1Gb/s. You should go into properties for the NIC device in Windows Device Manager and change the Speed & Duplex from "Auto Negotiation" to "1.0 1Gb/s Full Duplex"

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