Task Manager has always been my first reflex when something feels off. If my PC slows down, I press Ctrl + Shift + Esc, glance at the Memory column, and see that the RAM usage sits at 80 or 90 percent ...
High CPU activity on a Mac can be fixed by identifying resource-hungry apps in Activity Monitor and quitting them, disabling unnecessary startup items, uninstalling unused apps, restarting the Mac, ...