Mac Mini 2012 incredibly slow

Question

Hi everyone,
I'm trying to fix a Mac Mini (late 2012 model) for a familymember. He bought a used Mac Mini and it looks good on the outside, but something seems to be wrong inside. Now, I'm great with computers but relatively new to Apple hardware, and so I turn to all of you for some help.
The symptoms:
- Mac Mini loads everything insanely slow. It's like a handicapped snail.
- Very random boottimes, sometimes it boots in 40 seconds, sometimes it takes 5+ minutes.
What I've tried:
- Checked the harddrive with the Disk Utility. The test returns no problems. The SMART status is also listed as verified.
- Ran a speedtest on the harddrive to see if its performance is below expectations. Results were around 100MB/s sec write and read speed.
- Ran the Apple Hardware Test: no problems detected.
- Formatting the harddrive and reinstalling OSX 10.8 Mountain Lion. Faced lots of problems during installation, had to start over three times (confirms it as a hardware issue for me).
Now, I'm thinking the harddrive is dying but all the tests show it's just fine. Any suggestions?

Answer 1

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider.
Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional — ask if you need guidance.
If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.
Keeping your confidential data secure during hardware repair
*An SSD doesn't need to be zeroed.