Upload bad on desktops but not laptops

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The original was posted on /r/homenetworking by /u/blazezx1 on 2023-12-07 05:09:38+00:00.

Need some help figuring this out.

Download on speedtest.net always tests right, but upload tests like shit, unless I test from my laptop or my boss' Macbook on the same ethernet cable as the desktop testing badly.

It ramps up to full speed for a second then drops to under half.

I have tried:

• Other computers
• Driver updates
• pcie cards
• usb adapters
• testing somewhere else
• connecting the computers up at another premises and testing there(they test fine there)
• cat5 shielded and cat6 cable.
• testing the desktop straight on the fiber ont making it do the authentication.
• lastly, tried the TCP optimizer from speedguide.net

I am somewhat at my wits end here.

Any help would be appreciated.

Edit:

Forgot to add these that I also tried

• safemode with networking
• ubuntu server testing the same
• fresh windows install