I have only one LAN integrated on motherboard. I have tried a LAN dongle plugged into USB-C, but same results.
When the speed is down on PC due to running MSFS, I can still have 1Gbps up/down speed on other LAN port of router. I will try and check changing router LAN ports.
Thanks for the help anyway!
Thanks for your feedback. I would say I am an a bit above average user, but far from being able to analyze network log files and logs and that’s why just trying to solve the problem with trial and error approach. If nothing helps, then I really need to ask for some professional help.
Have you tried the mac plugged into the same port as the one you are calling Office that your desktop PC is in?
Sounds like you have an RJ45 socket in the wall there, are you sure it’s wired with cat5 e or cat 6 back to the router?
Yes, I did. In the same wall socket the Macbook performs good, almost achieving 1Gbit up/down.
The hause is built last year, all cable is min. Cat5E or 6.
a different process use the 3-4mbit… You sorted already by “network traffic”, sure the “first line” was shown ? As @blueline308 mentioned - it is only 3mbit - but on my pc I wanted to know what process cause 3-4mbit traffic.
The hint with the cable is good. example: I have 10gbe and mostly it worked, but had some strange bandwith ranges while transfering from/to NAS from time to time. I changed the cable and got 100% stable 10gig. I opened the cable plug and at one of the flex ( wire in it ) was the isolation damaged.
I also see “realtek” ( hust )… Can you check in devicemanager the driver date ?
But , as you mentioned if you get the same results with a usb dongle, then may be router is the bad-boy. I remember msfs cause some broadcast messages, may be the rounter dont like that.
But then… why should the Mac run fine in parallel 
Have you measured the speed at the Mac while MSFS is running ?
Thus, may be back to driver issue ?
Have you als already checked the windows event log ?.. sometime are infos logged, e.g. in my case “Link lost” messages.
Yes, the list is sorted by network traffic, but independent of sorting (ascending/descending) Task Manager shows 0 traffic for all application what is clearly not true.
My suspision is somehow Win11. Are you guys already on Win11?
I have speed issue only with this PC, even when MSFS is not runing the speed is (especially upload) is way less than on my Macbook when I connect it into the same RJ45.
When MSFS was runing I’ve been able to have a 1Gbps speed on my Macbook connected directly into router.
I can start to play around and check cables, but as mentioned, with same cable other device performs well. 
Tomorrow morning an IT guy will remotely will have a look on my system. 
no, still on win10… in point of OS I will not be a beta tester 
Sounds realy strange. Only idea for me is still a driver thing, may be also more probably because of win11.
But… let us informed what the IT guy tell 
As I mentioned earlier I am on Windows 11 and it’s working OK apart from one of my NIC’s (the killer networks one) is not performing well.
The realtek one works fine and I get the same performance as I did on Windows 10.
Unfortunately no progress. The IT guy sad, to isolate the issue it would be needed to have a new Win10 installation on an another NVMe drive to swap or backup existing Win11 and install Win10 and later recover the backup. With this it could be isolated if it is hardware or software issue.
Honestly I do not feel ready for a complete reinstall…I still have all the bad memories from FSX, P3D era…where all addon, config, etc needed to be tweaked manually 
For now I put on hold this issue, have no more idea 
the IT guy will compare a win10 installation with your win11 installation ?.. I call that try-and-error and not knowledge 
I mean you tried already a usb network stick and get same behavior , thus hardware seems a little bit unlikely. But issue from software side,… hmmm… strange…
Agree…but what can I say…? I gave a chance to him.
I had some wifi issue with my router, reported to ISP and they sad if it happens again they will change my router to an another type…now I am hoping the issue will happen again and I can have a new router…i do not expect it is an issue for the PC speed, but have no idea, so I keep trying evrything
Now I have a new router (different brand) and the issue is still existing. Now I am taking deep breath to do Win11 reinstall, but before I did a quick test:
1.Microsoft Edge
2. Chrome
3. Speedtest Desktop App
Someone can explain to me why only the “Microsoft” Edge is measuring acceptable speed? The three measurements been done in a minute.
Based on this result I doubt if a Win11 reinstall would solve the problem. As shown by Edge, the speed is there, but looks like Win11 “decides” which app can access it…and there are more favorable ones
Any recommendation? Shall I still try full Win11 reinstall?
If I see it correctly, different severs are used for each test to measure speed. Maybe the speedtest is faulty?
Yesterday I have reinstalled Win11, at night I left my PC to install MSFS.
After every piece of software I do a speed test in Edge and in Speedtest desktop app…so far both shows app.900/900Mbps down/up speed. 
Still long time to go to install all addons and setup graphics and control.
After so many trials, it looks the win11 reinstall seemingly solved the issue.
I have consistent measurement with Edge and speedtest desktop app.
Pleasant surprise, all my MSFS settings were saved under my live account, so no need to set it again after a full reinstall.
It is bad I had no idea what caused the issue, but it is good working now and can fly the PMDG in less than an hour the fenix A320 as well
I am glad you finally got it sorted out !
I have the same problem 130Mbit/s and starting MS2020 it slowsdown to 10Mbit/s. Stopping MS it stil remains 10Mbit/s. After restarting the computer i have the 130Mbit/s back.
So I sould reinstall window11 ?
I have the impression win11 wants to prioritize application which it thinks needs the band with. My issue is slowly but surely coming back, at the moment “only” after windows startup applications can’t reach internet just about 30sec after startup. Anoying, but can live with it, I do not want to reinstall again windows and MSFS 
Me neither. I reinstalled FS2020 a week ago and due to the bandwith of 10 Mbit/s it took me 2 days
Using Legion 7 with “Network Boost” ON gives priority to games, which caused very slow d/l to other apps including ‘Speed Test’. Turned off Network Boost and all is well. Maybe you have a similar setting.
