Are you the ONLY person at the house? an NO other device on the network? no smart speakers or TV, Roku, Apple TV, etc. And no Wifi.
I'm guessing no
in which case YOU HAVE to CHECK the NETWORK. This has nothing to do with what you PC is doing (though it could be, malware is possible though not necessarily likely). The question is what is EVERYTHING on your LAN (that you expect and that you don't) doing in terms of traffic?
And then you need to test. A cloud sync was one possible test method, not the only one. After setting up your network such that NOTHING else is using it, testing sustained upload bandwidth checks both your router and your ISP. How you do such a test (over many minutes, as in 10-to-30+ minutes... sustained) is up to you
As for the plugin... prior experience doesn't count. that plugin talks extensively over the Internet. something that was okay may not be now. Others have disabled/uninstalled it and seen issues clear up. Others are fine with the plugin. Its quality is highly suspect. so your mileage may vary, so to speak. The plugin could be hanging up on something remote and that is having a trickle down effect on the Operating System/NIC. Or... lots of possibilities