Light-weight system monitor tool/app for OSX, Conky alternatives
MenuMeters


Official website, free &amp; open-source
A fork of MenuMeters is being maintained for El Capitan+ including Mojave and Catalina: Maintained Fork Repo




Features:


CPU, memory, disk, and network monitoring tools for Mac OS X;
are a true SystemUIServer plugins (also known as Menu Extras);
Net Meter can display network throughput as arrows, bytes per second, and/or as a graph. 
minimalism: extremely lightweight and dumb simple;
size is ~1Mb;
open-source, freeware.


Official website states about exception:


  MenuMeters cannot be used on 10.11 El Capitan


But I managed to run on OS X El Capitan (10.11.3) without any problems, I will provide the link on re-compiled version, as this tool is opensource. It works like a charm for a last year, not a single issue found even after automatic system updates.

Configuration screenshot: