These are the steps I would generally recommend:

 1. Backup
 2. Run Disk Utility from the Recovery Partition (press cmd+R at startup)
 3. Run [Onyx][1]
 4. Reinstall OS X (e.g. from recovery partition)
 5. Clean reinstall OS X (i.e. wipe partition, install OS X, restore backup)
 
After each step, see if you get better results. 

In your case I think you have a very decent machine and it shouldn't really be much slower than current MacBooks, especially since you already have a SSD. 

However, your SSD is too full. A SSD needs ideally about [1/4 of empty space to maintain performance][2]. Also OSX is known to slow down on full disks. In your case step 5 may be necessary, because maybe the SSD couldn't effectively do garbage collection or TRIM. By overwriting the disk empty blocks get reinitialised. You can try [activating TRIM][3] in the future (if using a non-Apple SSD). 


  [1]: http://www.titanium.free.fr/onyx.html
  [2]: http://www.anandtech.com/show/6489/playing-with-op
  [3]: https://apple.stackexchange.com/a/181673