Linux system info

/proc/cpuinfo and lscpu have all the info.

As far as I know, physical processor refers to a real, single physical processor, not a processor core. And virtual processors are cores times the number of threads on each core (I think?).
See Counting processors on your Linux box | Network World.

To get CPU load you can use top. Or /proc/stat. Example: bash - How to get overall CPU usage (e.g. 57%) on Linux
Or even better, /proc/loadavg.

free - display amount of free and used memory in the system (parses /proc/meminfo). -m to print in mebibytes, --mega to print in megabytes.

df - display disk space usage.
--block-size=MB for megabytes, --block-size=M (or MiB) for mebibytes.

Show 20 most space-consuming files (or folders):

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du -h | sort -rh | head -n 20
# or this, to avoid comparing subdirectories:
du -sh * | sort -rh | head -n 20
# or, to include hidden files
du -sh {*,.[^.],.??*} | sort -rh | head -n 20

About this arcane {*,.[^.],.??*} stuff. Don’t know if works in fish.

You can also try this:

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du -sk * | sort -rn

netstat all the way. netstat -n for resolving hostnames fast. Helps on a VM.