Dell Inspiron 15 3558

I just purchased a new Dell Inspiron 15 3000 series laptop from best buy two days ago. The laptop was slow on first boot and I thought it was normal since it's getting everything together first time use. The laptop was pretty fast for a while. I downloaded a few things and installed Titanium internet security. I also made a 75GB partition and created a system image. After a while, I noticed that my laptop took a long time to boot(~200s) and when it did boot up, the search bar and menu would take some time to work after I clicked on it. However , I was able to play the games i downloaded pretty well. Today, the same startup problems remained. When I went to play Blade & Soul, my laptop froze on the launcher several times and I had to force shut it(hold power button) to get my laptop to work. I set the compatibility mode to Windows7 and the problem was resolved with freezing during launch. However, every time I play Blade & Soul and exit the game, my computer becomes really laggy until I restart. I also tried Blacklight: Retribution and it was surprisingly laggy in game and my laptop became slow after exit. I ran disk defragment and it didn't do much good. Every startup, my laptop is slow. Every time I exit a game, my laptop is slow. I just bought it yesterday brand new so why am I getting all these problems?

The laptops stats are:

5th gen i3-5015u (2.1 GHz)

Intel HD 5500 graphics

4 GB RAM

1TB HDD