Speed difference by putting the external SSD (connected to TB2) to Internal ?

Welcome to Apple Support Community.

Assuming the Thunderbolt connection is in good working order, it would not be any faster to install the SSD internally. Thunderbolt is already rated faster than the internal SATA connection. Plus, iMac was never designed to repaired or upgraded by the end-user, and these modifications have great risk of damage or permanent failure to the iMac. 

Jack

I agree w Jack-19.

As you indicate you have read, there would be nothing to gain by installing the SSD internally to your iMac. That's true.

And, as you probably already know, the risk of damaging some other internal component is high should you open up the computer.

Stick with using the SSD externally.

Also, if you are still booting the computer from the internal drive, I suggest you make that external SSD your startup drive. That would def make a difference in performance.

you can look up details specs of Mac models here, so that you can actually see what are the speed ratings of the various ports and internal interfaces,

https://everymac.com

for example, in the 2014 iMacs the internal SATA6 (6Gbit/s) is no match for the external port Thunderbolt2 (20Gbit/s) speed.

There were older Macs before Thunderbolt, that the external ports were limited to USB2 and FW800 however, like way back in 2010, where the internal SATA3 would have been faster, so that is a good question.