Some things to maybe try looking at/ruling out:
Possibility #1: Your router is
dual band (5Ghz and 2.4Ghz) and the desktop for some reason is connecting on the other band from everything else (older wifi cards only do 2.4Ghz). Or if your router is crazy new it could be tri-band, similar deal.
If, say, the laptop and stuff are on 5Ghz and the computer is on 2.4Ghz, then there could be some new piece of technology (a neighbor got a new router that's on that band, or a new smart device, or something) interfering with the 2.4Ghz band and it'd only cause the desktop to slow down. Alternatively, perhaps the desktop used to connect to the same band as everything else and has flipped to another one for some reason.
If your router has different SSIDs (network names) for the 2 bands then it should be easy to see which one your desktop is connected to. If it doesn't, you might consider separating them out so it's easier to force the desktop onto the correct one. Or try futzing with the channel on the 2.4Ghz band if it's currently auto-selecting, in case it's on an overloaded channel.
Possibility #2: There's a specific thing blocking signal to the desktop location. Have you tried moving it 3' in each direction to see if speed is affected?
Possibility #3: There's an antenna wired or plugged into the desktop wifi that has either become unplugged/cable broken or the antenna has fallen down behind a filing cabinet or something dumb like that.
Possibility #4: A bad driver update fucked over your performance on the desktop.
Possibility #5: The hardware is beginning to die.