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A slow Mac is rarely slow for one reason. It's usually a small pile of ordinary things: a nearly full disk, too many login items, a browser with 61 tabs, and a background app that has been quietly indexing since March.
Start with storage
macOS needs breathing room for swap and snapshots. If your disk is above 90% full, performance can suffer before you notice anything else. Use System Settings → General → Storage to see the big categories, then remove or offload the largest safe items.
Check login items
Open System Settings → General → Login Items. Disable anything you don't recognize or don't need at startup. This is often the fastest visible win.
- Restart your Mac. It sounds trivial; it clears weeks of accumulated state.
- Update macOS and your most-used apps.
- Look for browser extensions you installed for one task and forgot.
- Check Activity Monitor for apps using more than 100% CPU while idle.
Then clean up
A safe cleaner should show you what it plans to remove and let you skip categories. The good ones do exactly that. Avoid any tool that invents problems or insists you need to clean your RAM every hour.
The goal isn't a perfectly empty disk. It's enough room for the system to do its job.