I was in the exact same situation with what sounds like the same machine. I purchased a 1TB SSD and an USB3 enclosure on Amazon, migrated the spinning drive to the SSD, and have been amazed at the difference in speed.
To give you a couple examples: Safari now only bounces once on launch. Excel bounces four times.
Here is a link to my original post. (I did not enable TRIM.) OS on external SSD, leave data on spinning drive?
Steve