External Bootable SSD Late 2015 iMac

USB-3 will be fine, so just get a SATA to USB-3 adapter and an appropriately sized SSD, clone the internal to the external using CarbonCopyCloner and you’re good to go.

 

Buy a USB 3 SSD. Format it as APFS, install MacOS then use Apple's migration assistant to move you data and apps. Then change the default boot drive and reboot.

After using your new system long enough that you are sure there are now problems. and ADTER the first full Time Machine backup, then, erase the Mac Internal drive, make it also APFS and use it for storing bulk data that dose not need to be on the SSD.

The above is exactly what I did with my iMac. Also if you have no already done so install as much RAM as will fit the the iMac, just max it out.

YouTime machine backup drive does not need to be USB 3, it can be slow but you may as well use USB 3. Make sure this TM drive is at least 2X larger then al to data you have or will have

I would not erase (aka "reformat') the internal drive until after you are running a few data on the external SSD and Time Machine has finished it's first complete backup of the new SSD. Maybe even wait until our cloud backup is done? (If you don't have this, sign up for it. Backblaze is only $5 per month.)

One issue is keeping a usable backup while you make the switch. You have your old data i-on the old drive and I presume a Time Machine backup of this. To avoid accidents, unplug the tm drive and lock it in a drawer until after the transition is complete. This will prevent human error which is easier than you r think to do. Put the old TM drive is your desk at work, or see place far away. Buy a new TM drive for your new setup.

 

While Thunderbolt is potentially better, price and availability gives USB3 significant advantages and only a small performance downside.

 

I bought a cheap TB1 used drive and replaced the internal spinner for an SSD to use as the boot drive for my sister's 2015 iMac. The internal drive was a very slow 5400rpm. It wasn't that expensive to do. The main advantage is that TRIM is supported over TB but not over USB.

 

Was in the same boat as you. Went with USB 3.0 enclosure with UASP support. What an improvement! About 370mbps/370mbps speeds versus 100mbps before. I suppose that's about 25% slower than Thunderbolt but well, you just can't notice it at all. The differences are imo not worth the cost of TB enclosure.