I m sure there are several, but I use Gnome System Monitor. Clicking the resources tab displays: processor usage for each cpu thread, memory and swap usage, and download/upload usage.
Gnome-system-monitor, comes by default for most ubuntu- and gnome-based linux distros. Same thing as the Task Manager, basically.
FWIW, shift+f3 in-game brings up a pie chart which you can navigate to see what's eating up the fps. And there's Opis as well.