A phone that was fast and is now slow usually has one of four causes. None of them is fixed by an app from an advert.
1. Storage is nearly full
This is the big one. Phone storage slows down dramatically when it is almost full, because the phone has nowhere to work. Under about 10 percent free, everything crawls.
Check Settings, Storage. If you are close to full, that is your answer, and the storage guide is the fix.
2. Too much running in the background
Every app you install can wake up, sync, check for notifications and hold memory. Thirty of them together makes a cheap phone feel broken.
Go through your app list and uninstall what you genuinely do not use. Not to free space, but to stop the background work. Be honest about the ones you installed for a single task and never opened again.
On Android you can also restrict background activity for specific heavy apps: Settings, Apps, the app, Battery, Restricted.
3. It needs restarting
Many people never switch a phone off. Memory fills with things that were never cleaned up. Restarting once a week costs a minute and makes a real difference on a phone with 2GB or 3GB of RAM.
4. It is genuinely old
Apps grow. WhatsApp and Facebook today need considerably more from a phone than they did five years ago, and a phone that was entry level then cannot keep up now. That is not a fault you can clean away.
If it is this, two things still help: use the lightweight versions where they exist, such as Facebook Lite or Messenger Lite, and reduce animations, which makes the phone feel faster even where it is not.
Android: turn on Developer options by tapping Build number in About phone seven times, then in Developer options set Window animation scale, Transition animation scale and Animator duration scale to 0.5x or off.
What not to install
Do not install cleaner, booster or RAM optimiser apps. They cannot do anything Android does not already do, they take memory and storage themselves, they show adverts, and several popular ones have been caught harvesting data. A phone with a booster app installed is slower than the same phone without one.
The last resort
A factory reset genuinely does restore a phone to its original speed, because it removes every accumulated app and setting. It also erases everything.
Only do it when you have a verified backup, when you know your Google or Apple password, and when you have a plan for WhatsApp. Work through the backup guide first, then Settings, System, Reset, Factory data reset.
Afterwards, reinstall only the apps you actually use. The speed lasts exactly as long as that discipline does.