It's easy to set up an external USB3 boot drive, and it will be MUCH faster.
First, you'll need the external SSD.
Samsung t5 might be good, if you can get one.
Or, something like this:
http://www.amazon.com/exec/obidos/ASIN/B00ZTRY532?tag=delt-20
Or, you could even use a "bare" 2.5" SSD with an external enclosure or even a USB3/SATA adapter like this:
https://www.amazon.com/Sabrent-2-5-...478&sr=1-2-spell&keywords=sabremt+usb3+to+ssd
MAKE SURE any external enclosure you buy is advertised to support "UASP"!
You DON'T NEED to buy a 1tb SSD.
500gb is fine.
You can even do with 250gb.
You can still use the internal drive for extra room (of course).
Once you have the SSD, use Disk Utility to format it for HFS+ with journaling enabled.
Then, I suggest you download either CarbonCopyCloner or SuperDuper.
Both are FREE to download and use for 30 days, and will do what comes next.
1. Open CCC
2. On the left, put your source drive (the internal drive)
3. On the right, select the target (the SSD)
4. You can accept CCC's default choices
5. CCC may ask if you want to archive and copy the recovery partition. YES, you want to do this
6. Let CCC run and do the job.
When done, you'll have an exact copy of the internal, now on the external ready to boot.
Now, restart.
Hold down the option key right after reboot, and keep holding it down until the startup manager appears.
Select the external drive with the pointer and hit return
You should now boot from the SSD
When you get to the finder, go to "about this Mac" (Apple menu) to be sure.
One more thing to do:
Open the "startup disk" preference pane and set the SSD to be the boot drive.
Now the iMac will always boot from the SSD.
Really, it's very easy, once you get these tasks done.
. It totally makes me satisfied except its HDD 
