I keep Activity Monitor in my dock for this reason. You can right click its icon and change it to be a real-time graph of a few different stats. I have mine set to CPU usage, it’s like a one-metric iStat Menus for the Dock
Then I can click it to open a dropdown that ranks the top 5 processes of each category (CPU, MEM, NET) and the amount they are consuming.
It's this kind of thing that should be built in to the OS. Makes it trivial to eyeball whether something unexpected is going on.