2 computers with vastly different upload speeds on the same connection (but same download)

Hey, looking for some help figuring something out.. been at this problem for several days now, but i'm not a network guy so couldn't find a solution...

I have 2 PCs in my local network, call it PC 1 (main with 2.5 GbE port) and PC 2 (minipc with 1 GbE port), both are hardwired to the router and both run Win11 pro. both PCs, when I do speed test to a local server (i.e. within the country), get max speed (I have a 2Gbps down 1Gbps up plan, so PC 1 is getting 2gb down and 1gb up and PC 2 is getting 1gb down/up) but when I do a speed test to a different country, with PC 1 I get severely reduced upload (up to 80% less in some cases - 200Mbps) compared to PC 2, which gets near max, download is almost unaffected in both cases.

The worst case I recorded was PC 1 getting 200Mbps upload vs PC 2 that gets 950Mbps upload (download speed is within 100Mbps of each other)

The test was done with speedtest.net mostly, but I also tested using other services (Netflix, Google, Cloudflare, etc), results all showed that PC 1 has much lower upload speed.
I tried those tests in Firefox, Chrome, and Edge

I cannot figure out for the life of me why both PCs will get max speed in local speed test (to the same server!), but vastly different results when testing to further away servers (again, to the same server).

This is what I tried:

* Tested with another ISP
* Swapped ports on the router
* Tested with only 1 PC plugged into the router at a time, and tested all ports
* Swapped router
* Swapped cables (I got cat 5e and PC 2 is actually further away than PC 1, but at most we're looking at 10-12 meters)
* Updated drivers
* Downgraded drivers
* Copied network settings from PC 2 to PC 1
* Tested with a 3rd PC, which also had reduced speeds like PC 1.
* Tested with Android phone (this was done with 5g wifi) - got max speed down/up no matter the server.
* Did tracert on both PCs to several speed test servers, all were identical.
* Tried safe mode /w network on PC 1 - wasn't better.
* Tried with a different NIC, as in, different chipset - no change.
* changed countless settings under the Advanced tab of each NIC (where you have Large Send Offload and Speed & Duplex, etc).
* made a VM on PC 1 with a win11 pro install and Ubuntu to test within the VM, the win 11 VM had extremely bad speeds, both down and up, while the Ubuntu showed better upload than PC 1 by about 50%.

I later on got my hands on a realtek NIC (8111c) and tried some more tests:

* I booted Linux Mint from USB on PC 1, and there I saw something different... Tested to a local server both NICs had max speed (as before), but tested to a further away server, the realtek NIC had almost max up/down speeds (~850Mbps to far away servers) and with the intel NIC I had about 50% uplift with the upload.
* I had a spare SSD so decided to install a fresh win11pro copy and... the speeds there were much much worse to further away server: download was pretty max but my upload was stuck at 20-25Mbps (compared to the 200Mbps on OG OS).
Speeds to local server were fine again (i.e. max in both up and down).

I even took PC 2 to where PC 1 is, connected it to same power splitter and plugged it using PC 1's ethernet cable and still the same: when PC 1 is connected with the cable, i get lowered upload speed, when PC 2 is connected i get max upload speed (download speed is fine). PC 2 (the one with the faster net) is usually sitting behind a dumb switch, so I tried putting PC 1 behind that same switch, no change.

There's no packet loss, the ping is the same...

Here are the specs:
PC 1 has an MSI pro z790-p wifi mobo with Intel i225-V rev03 (2.5 Gbe) controller, and PC 2 is a mini pc (Beelink S12 Pro) with a Realtek GBE family controller.
Both PCs run Windows 11 pro, version 24H2 (OS build 26100.4061)

So to recap: 2 PCs, both cap net speed when tested to a local server (within the country), but when tested to a foreign server, PC 1 sees up to 80% less upload speed compared to PC 2. Download speed is mostly unaffected.

At this point I'm pretty sure it's not a hardware issue.
Also, every test I've ran at least 5 times before moving to the next one (and always tested the same servers when doing local vs remote).
The linux mint test really confused me...

Any, and I mean ANY help is appreciated.

Sorry but there's little I can add that might be helpful but did want to welcome you to the forum and commend your thorough troubleshooting and excellent explanation. You have certainly gone above and beyond in your efforts.
It is a very weird issue so if you are able to solve it, please post back the solution. We have a lot of experienced folks here so I'm sure someone will offer some suggestions.

glasskuter said:

Sorry but there's little I can add that might be helpful but did want to welcome you to the forum and commend your thorough troubleshooting and excellent explanation. You have certainly gone above and beyond in your efforts.
It is a very weird issue so if you are able to solve it, please post back the solution. We have a lot of experienced folks here so I'm sure someone will offer some suggestions.

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Thanks, this unexpected adventure is slowly causing me to descend into madness