Ridiculously slow MacBook Pro I have a MacBook Pro (I think mid 2012). About three months ago it started running really slow, so I renstalled the OS with a recovery partition. For a few weeks it ran very well with an overall very seamless experience. 

Yet, as of two days ago, suddenly it started running extremely slow again. When I use vim and save a 200 line text file it takes about a minute average. When I want to click an icon or menu bar, it's not unusual to delay even highlighting the widget for 30 seconds. 

I thought it might be the hard disk drive, so today I changed to a brand new SSD. This, however, had no effect.

The computer runs at this very slow speed even when system monitor says that there are 1.5GB RAM left and 97% idle CPU.

Does anyone know what the issue might be?

Right now I’m trying to run the recovery partition again, yet even booting from it has already taken me longer than writing this.

Edit

I have only used about 30 GB of my startup disk.

I am not aware of any malware, yet, I canot be sure.

I have 4 GB of RAM (DDR3 1600MHz)

CPU is i5 2.5 Ghz

Update

I got it into recovery mode, and even here it's painfully slow, even if I select a disk i disk utility, it takes quite some time to even highlight the name. Could a reinstall even do the trick here?

More clear details

Computer specifications:

MacBook Pro 13" (mid 2012)

CPU: 2.5 GHz i5

RAM: 4 GB 1600MHz DDR3

Graphics: Intel 4000

Disk: 500GB (now 120GB) Only 30 GB when running slowly.

No retina.

OS: Mavericks, it came with Mountain Lion (It has run well with Mavericks)

SOLUTION

As suggested I tried to replace the Hard Drive cable following this post.

The cost of this was the cable itself (around 32 EUR), and was done with only PH0 and PH00 screwdrivers. 

It's been working perfectly for around 30 minutes with no sign of the prior slowdown. The repair was very easy and the effect is just as one could wish for.
Hard drive cable failure is almost guaranteed to be your problem. We have a deployment of around 70 2012 13" MBPs for staff members, of which around 10% have had a hard drive cable failure. I had a suspicion you were going to say you had the same model from the symptoms you described.

The symptoms on our problem MacBooks are exactly the same as yours with the MacBooks running excruciatingly slowly on the most basic of operations. Replacing the cable has restored full speed without any further work needing to be done. Simply re-seating the cable's connector on the logic board has worked on a couple of occasions - in the short-term - but these have generally slowed down again, sometimes completely failing.

The odd thing is some of these MacBooks haven't been subject to any abuse and the cable has showed no signs of damage. However, the simple fact is that replacing the cable has worked.

The cables are available online (genuine cables can be had at a reasonable price on a certain auction site) and can be changed in under 10 minutes. The fiddliest part is disconnecting and reconnecting the IR receiver/status LED as there's a tiny latched connector to undo, but otherwise it's an easy fix.