Defrag is not going to be a big help, and if you do see an improvement, it will eventually go away. The only safe defrag is a backup/restore, except for the risk that you data will get lost in the process because one of the devices involved dies.
As suggested by CT post the EtreCheck output. Chances are you have lots of crud on your system that is contributing to your slow down. Yosemite is very sensitive to 3rd party curd installed during previous OS X generations. Even good 3rd party stuff that has not been updated to a Yosemite compatible version.
As suggested by CT and Csound1 try not to keep your disk full. When the free space gets too low, the file system spends a lot time looking for where to put files (and with very little free space, any defragmentation you do will be quickly lost by normal disk activity).
Your system is 5 years old. It is also possible that your rotating hard disk is starting to generate errors that cause the disk's controller to do lots of retries. A consumer disk may spend 20 seconds attempting to recover a bad sector. If you disk has degraded so that there are enough of those bad sectors your performance can get really bad. NOTE: I'm NOT saying your disk is bad. I'm just saying your Mac's age puts the disk in the ball park where a disk can start to fail. I've got a 2006 Mac mini with the original disk that is still going strong, so a failing disk is not s sure thing. I've also had system where the disk went in less than 2 years.
Once crud is removed, and you have removed as much unnecessary files as is reasonable, then start to think about upgrades. RAM is one of the best performance boosters. 8GB would be good, 16GB would be better, assuming you actually do things that would benefit from RAM. Again the EtreCheck output would help us advice you better.
To really trick out the Mac, replace the rotating hard disk with a Solid State Disk (SSD), or keep your hard disk and remove the DVD drive replacing it with an SSD (there are conversion kits and instructions/videos/etc... available - see MacSales.com for one).
I replaced my hard disk with an SSD and put 16GB of RAM in my Late 2011 15" Macbook Pro. It is a very nice system with good responsive performance.