Have you checked the signel strength on the slow PC - just hover over the connection icon and it should tell you. It may be that you have a weaker signel to the slow PC due to walls or something like that.
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Consider checking the adapter in Network Connections. You will porably find in the advanced tab some controls with performance tuning (for example a slider or drop down list for choosing between longer battery life or better performance).
Also ty changing channels on the router, it could be some interference in the area this computer is located.
And make sure you test this with any "Turbo" mode off, to make sure the router itself is not having problems.
Since this is a PCI card, open the Device Manager and check if there's no yellow marks next to it, indicating conflicts with other hardware on this computer.