I have never owned an Apple product, and I got a MacBook Pro (13-inch, Mid 2012) with OS X El Capitan ver. 10.11, about 5 months ago, for work purposes. It's not using SSD yet.
Suddenly, one day, it slowed down. Like when you're opening too much applications, your cursor would show loading icon anytime you're clicking on something (but it can still move around freely and smoothly). To the point the song iTunes was playing is lagging. The I restarted it. After some hours, it happened again, and I restarted it again. This time, it stuck on OS loading screen (where there's a gray apple with loading bar below). Restart 2-3 times it using ON/OFF button, failed, stuck on the same screen.
Then I tried to go into safe mode, and succeed going into desktop. After that, I restarted it again and successfully got to the desktop normally. But now, it would slowed down (cursor turns to loading icon every time you click on something) very often, around every 5 mins. Even more, some apps I frequently used (that I used when symptoms appear) cannot be opened, such as Chrome and Eclipse. I haven't even run IDE and emulator and it's like that already.
All this time, usually after I was done with my work for the day, I would just close the lid and didn't shut it down. Some people has said that "It's not SSD, so you should not do that often", while someone that I know, a user of MacBook too, said "Ah, I got no problem doing that with my Mac, which wasn't using SSD". Is that really the problem?
I've upgrade the OS to 10.11.5, but same thing happened. Also, I didn't found Apple Diagnostics or Apple Hardware Test. I've run Disk Utility's First Aid too, and it only said "Operation successful", "volume seems to be OK".
The lag happened so suddenly, where yesterday it really did just fine and suddenly today it broke down. Does anyone have any idea about what was really happened and what should I do?