Ubuntu 15.10 runs very slowly on my Dell Inspiron laptop

I bought a new Dell Inspiron: laptop.

Processor: i5-6300HQ HDD: 230GB 5400RPM RAM: 8GB NVIDIA GeForce GTX 960M 4GB GDDR5 

I am dual booting Windows 10 and Ubuntu 15.10. For some reason Ubuntu appears to be running very slowly, even slower than when I used Ubuntu on my dual core 4GB RAM laptop. Is there anything I can do about this? I am already running the version 352.63 NVIDIA graphics driver. I installed it from the Ubuntu Software Center's Additional drivers tab.

It takes a lot of time to boot and also the applications take too long to open and respond slowly.

Attaching a screenshot from SMART test.

^{(Click image to enlarge)}

Is the problem only related to slow hard drive, or is it because of some other reason too?

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