Helpful Cheatsheet if you need to do some digging
While you can get a pretty good overview in Windows using Perfmon, it might be a little bit harder in Linux.
Have to admit that i've never tried half of the tools on this image, and since netstat got replaced by "ss", i fear it might also be a little bit outdated.
But it's still a good starting point if you have to find the bottleneck, as for example why Raspberry Pi's aren't the best NAS Servers.
