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You can easily eliminate one variable of your equation by temporarily closing the Dedicated Server Launcher after your server started (you can restart it after, it will just continue from where it was.
If the performance issue continues with the DSL shutdown, you know it’s not in cause.
I see that you’ve done some ping and tracert, which is what I would have suggested, so that’s good.
What I would do is to run the windows Resource Monitor (you can access it from the Task Manager -> Performance Tab -> Open resource monitor) and look at what appears on the top (when sorting by highest) in term of memory usage, cpu usage, disk usage.
Could be something as stupid as an antivirus scanning every single file on the machine, or some misconfigured monitoring software hogging the network.