Short answer: limited storage, too many startup programs, overheating, malware, insufficient memory or a failing drive are common reasons a computer becomes slow.
Start with the symptoms
Note whether the slowdown happens at startup, while browsing, during video calls or only when several programs are open. That pattern helps narrow the cause.
What a diagnosis should check
- Available storage and drive health
- Memory use and upgrade capacity
- Startup applications and unwanted software
- Cooling, dust and operating temperatures
- Operating-system and hardware errors
Repair, upgrade or replace?
An SSD, added RAM or software cleanup can extend the useful life of the right computer. If the hardware is too old or several parts are failing, replacement may be more sensible.