Activity Monitor can show you which apps are chewing up the most CPU resources, memory, disk activity, and network bandwidth.
Sometimes your Mac may not run as smoothly or as quickly as you expect. And the culprit may be a single application. A lone app can bump into performance issues, slowing down not just itself but your entire computer. An app may even freeze or become unresponsive to the point that you can’t close it the usual way. Whatever the problem, one tool that can help is Activity Monitor.
Built into macOS, Activity Monitor shows you a list of all running apps and processes and reveals the biggest hogs based on CPU, memory, energy use, disk use, and network bandwidth. And if a specific app is unresponsive and can’t be shut down the traditional way, you can force it to close. Here’s how it works.
To open this tool from Finder, click the Go menu, browse to Utilities, and then launch Activity Monitor in the window that appears. The utility starts by showing every running app and process based on CPU resources. Each column for CPU shows a useful result, but the stats for CPU percentage are the most helpful for app troubleshooting.
Read more at MacWorld.com