Whenever the performance on the computer has become slow, check for any viruses with an antivirus software check
If there's no virus then try to close out running/open applications on the computer to see if performance is better
Whenever the RAM memory is full or close to being full, the performance will be affected
Checking the Performance Monitor, details of the computer whether it's been turned on for multiple days without shutting down, applications being opened without closing, etc will be reported on why the performance is slow
In any case that the computer has been shut down, any applications are all closed including background applications, give the computer a break for fifteen minutes or so
Make sure computer has been updated (updated not upgraded) and see how performance is