Step 1: Free up disk space (biggest impact)
macOS slows dramatically when boot drive is over 90% full. Check: Apple menu → About This Mac → More Info → Storage.
Use Storage Settings to remove large files. Empty Downloads, clear Trash, remove unused apps. Aim for 20%+ free space for SSD trim to work properly.
Step 2: Manage login items
System Settings → General → Login Items. Disable anything you don't need at startup — Spotify, Slack, Adobe Creative Cloud, OneDrive sync agents all add 30–60 seconds to boot.
Background Items section: review apps with constant background processes (Dropbox, Google Drive, antivirus). Each one taxes CPU and battery.
Step 3: Reset SMC and NVRAM (Intel Macs only)
Intel Macs: Power off. Hold Shift+Control+Option + Power for 10 seconds. Release, power on. Then for NVRAM: power on while holding Command+Option+P+R until you hear startup sound twice.
Fixes random slowdowns, fan-noise issues, sleep/wake bugs. Apple Silicon Macs don't have SMC or NVRAM in the same way and don't need this.
Step 4: Disable visual effects
System Settings → Accessibility → Display → Reduce Motion (ON), Reduce Transparency (ON). On older Macs (2017 and earlier), this alone gives a noticeable speed boost in Mission Control and window switching.
Step 5: Check Activity Monitor for runaway processes
Launchpad → Other → Activity Monitor. Sort by CPU. Anything sitting at 100%+ when idle is your culprit — typically a hung browser tab, antivirus scan, or Spotlight indexing rebuild.
When software optimization isn't enough
If your Mac is still slow after these steps, the issue is likely hardware: failing SSD, low RAM for current macOS, or thermal throttling. RAM upgrade (Intel Macs only) and SSD replacement are the two hardware upgrades worth doing.