Your system is running slow due to following reason which is given below
Your computer is too old
May be your hardware is damaged
Degradation of processor speed
Your CPU is over heating
Your Hard drive is Degrade
Your Run Registry is Full
Your Window is outdated
Malware,trojan or virus issue
Your Hard drive is full
Number of Temp file increase
You are not Reboot your system
This issue can be resolve easily you just need