Watch CPU, memory and temperature from the menu bar

macOS already ships with Activity Monitor, in Applications → Utilities. It lists every process with its CPU, memory, energy, disk and network use, and it is the right tool when the fans spin up and you want to know why. Sort by % CPU and the culprit is usually at the top. The Memory tab has the number that actually matters, Memory Pressure: green means you are fine regardless of how full the bar looks.

What Activity Monitor will not do is stay out of the way. For a permanent readout, Stats is a free, open-source app that puts CPU, GPU, memory, disk, network and sensor readings straight in the menu bar, and lets you choose which of them to show and how.

Install it with Homebrew if you have it:

brew install --cask stats

Either way, resist reading too much into a busy CPU. A Mac that is briefly at 100% while it indexes or exports is working, not struggling.