There are several reasons that computer performance goes down like Hardware issues, Software issues,Improper using behaviours, just to name a few.
not enough RAM, hard drive failing, outdated CPU, lack of storage space, malware/adware infection, Windows registry errors, third-party applications that over demand system resources, etc.
Click here: https://support.hp.com/in-en/document/c00034290 to know more about Improving PC Performance (the document might say Windows 7 however its the same steps for windows 10 )
Hope this helps! Keep me posted for further assistance.
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