Your Mac may be running slow due to an outdated macOS, too many running apps, low disk space, or an unorganized desktop. To speed it up, update macOS, quit unneeded apps, delete unused files like old email attachments, empty the trash, remove large unused files, delete old iOS backups and software installers, wipe unused language files, and organize the desktop. Cleaning cache files, uninstalling unused apps, and freeing at least 10GB of disk space can help improve a Mac's performance.