A device that felt instant when new feels sluggish after a few years. The hardware has not worn out. Several specific things have happened.
Software got heavier
Applications and operating systems expand with each version. The same machine runs code that expects more memory and more processing than it did when it shipped. This is the largest single factor.
Storage filled up
Solid-state drives slow noticeably when nearly full, because the drive needs free space to reorganise data. Keeping ten to twenty percent free makes a measurable difference.
Background processes multiplied
Every application installed over the years added something that runs at startup or checks for updates. Individually small, collectively substantial.
The battery degraded
On phones specifically, a worn battery cannot deliver peak current, and the system reduces performance to avoid unexpected shutdowns. A battery replacement restores speed, which sounds implausible and is well documented.
Thermal paste dried out
On laptops, the compound between processor and cooler degrades and dust blocks the vents. The machine runs hot and throttles itself. Cleaning the vents helps; replacing the paste helps more.
What actually helps
Free up storage. Disable startup items you do not need. Replace the battery on a phone older than three years. Clean the vents on a laptop. Reinstall the operating system if it is several years old, which clears accumulated cruft that no cleaning tool reaches.
What does not help
Cleaner and optimiser applications, which mostly delete temporary files that would have been deleted anyway, and which frequently add background processes of their own.