That's the part I do not fully understand - the CPU is not weak as we discussed before, it's a Core i7, albeit a laptop version in a relatively old Lenovo Thinkpad T520 with 8GB RAM and my Internet connection is 240/25Mbps cable - although I can get only around 180Mbps due to old router but that's more than good still I suppose.

I mean, it does not feel like it's broken, it's definitely syncync as the NAS is basically unusable now and working on the laptop is a pain. It's just slow. Extremely slow. Couldn't perhaps Windows Defender or firewall be interfering with the process?

I can also test it on a game rig I have access to (ex-BTC mining machine, though upgraded to a very fast core i7 4790K with 16GB RAM).