Using external SSD as primary boot on iMac how to implement?

Question

Iron

Please help with advice!
There is an iMac 21 junior model of 2013 , used mainly for work (printing design). About a month ago, the hard drive (HDD 5400rpm) began to crack strongly. And in general, after installing Yosemite, the system began to noticeably slow down. Disabling spotlite and effects didn't help much. The set of programs is standard and has not changed over the year (CS6: Id, Ps, Il).
I want to use an external SSD as the main boot, and the internal one is just for storing files. I read various articles in Russian and English on this topic and did not fully understand.
Does this decision make sense? How noticeable is the difference in performance?
USB 3 or Thunderbolt ?A ready-made kit or a separate disk and hub connection? What are the options?
Budget: 10-12 tr.

Answer 1

Why not put an SSD inside?
And for internal to buy a USB-SATA box?

Answer 2

I also want to try. They said that it’s easy, it’s better through Thunderbolt, you put the system on it from a flash drive, everything works. But I don’t know the performance either, purely theoretically, it can be increased by 2-3 times, but practically ... if anyone has tried it, please let me know.