How much faster is an external SSD than external HDD if internal drive is SSD?
In your case probably a upgrade to SATA 3 SSD (internal) would do the most.

AFIK your model supports it, and you will get up to 6 Gb/s R/W speeds and combined with RAM upgrade would get you what you want.

Adding a external drive to expand the storage if needed, the one would use the Thunderbolt SSD.

Find out if your Model supports the SATA 3 SSD before spending the money:

Open About this mac- more info- system report - SATA..

then you will see something like I have



My little MacBook Air rocks, it can run up to 6 Gb/s