Faulty Windows Operating System: A pirated version can result in Laptop slowness. A faulty version of windows.
Faulty Hard Disk : If the Health of the Hard disk goes below 100 percent by even 1 sector, then the disk becomes slow and gets into a failing state.
Malware: Malware can come in the form of software that is targetted to steal a device resource like CPU, Storage, Memory, RAM and data.
Viruses: When your machine is not protected by an antivirus, Viruses can affect and reduce your system performance. Virus will consume all the system resources and damage the OS and the Hard drive.
Low processer clock speed: CPU Clockspeed is the most expensive part of a laptop, It should be always above 2.00 Ghz. You can not replace your CPU so always check the clockspeed before buying.
Usage of HDD instead of SSD: HDD is an obsolete technology. It became obsolete in 2013 when SSDs were introduced. SSDs are 10x faster than an HDD.
Faulty antivirus: An antivirus has to be installed when a machine is newly formatted and not when it is already infected with a virus. Choosing a good Antivirus is very important.
Motherboard shorting: If there is a shorting on the motherboard it make the laptop slow.
Dried thermal paste:Thermal paste is essential when it comes to keeping the CPU and GPU cool. Using your computer with dried-out thermal paste will lead processor heating up and making your laptop slow and eventually causing it to go dead.
Laptop clogged with dust: Dust in quantity is a great insulator and retains heat, making your laptop slow and overheat. If the CPU fan of your laptop gets clogged with dust the fan will not be able to cool the CPU which will lead to CPU overheating and causing the CPU and board going dead.