I'll keep this simple and quick. 

        Last year, I bought this laptop (refurbished). It came with Windows 7 Ultimate & Intel Core i5 and I asked them to upgrade the ram to 8GB. From the start I have been experiencing slow performance, but it did not bother me too much. Around three months ago the laptops performance became very poor. I didn't take any quick actions since I had given the laptop to my dad and he didn't care about the slow performance as long as it did what he wanted, and neither did he report the issue to me. Recently I came back to using this laptop and have noticed the lags during games, no matter how much setting I change to low, or no matter how old the game is, as long as it is a game, this laptop will lag like crazy. Secondly the slow performance such as when I open two chrome tabs, one with a youtube video playing, the time taken to load the video is quite long despite the internet connection being completely fine. Some simple actions also fail to keep up a decent speed, such as refreshing (F5) on the homepage, it takes longer than a quick refresh should.

I tried a few things to see if it'll boost the performance-

Ran system diagnosis ; no issues found

Looked for viruses crawling somewhere ; still nope.

What could have caused the performance drop? Please help.