Thanks for all the help guys. On a side note how could I run an SSD ?
The least invasive method would be to use an external Thunderbolt or Firewire drive; there appear to already be multiple threads on that topic in this forum. (I'm toying with the idea of going that route myself with my 2012 iMac, which has an internal HDD.) The other option is to perform the proverbial heart transplant... that is, swap out your existing internal drive for an SSD. This method also leaves open the option to retain your previous config in its entirety, by moving the old drive into an external enclosure. (I actually did that with a 2006 iMac, many years ago.)
But frankly, I'm not sure I'd actually recommend
either of these options for you, since you already have a Fusion drive. I think you'd probably be much better off going back to that Apple tech to verify that your Fusion drive is working properly, because if the slowdown you're experiencing turns out to be unrelated to the hard drive, you might be setting yourself up for a very expensive disappointment, by just tossing money at an SSD. On the other hand, the tech should also be able to tell you if your drive is actually in the process of
failing... so they could potentially help you avert a very frustrating data loss event. (Hopefully you have a Time Machine backup, just in case.)
To put it another way: Unless you're already planning to entirely discontinue use of your Fusion drive for some reason, (such as age, as the most obvious example) buying an SSD is likely to only disguise the problem, while giving you very little back in terms of performance benefits. So, maybe try to find out for sure what's going on with that Fusion drive, first.