Internal SSD on iMac mid 2011

don't think thunderbolt would be a limitation in terms of speed

The 2011 iMacs had an option to have an internal HD and SSD when they were sold new, this means you could safely do this option. I made this change myself using the OWC instructions and cable kit to ensure the fans did not go crazy. If you are comfortable working inside an iMac it's not all that difficult, however look at the video (link below) and make that judgment. If the thought scares you, contact an AASP in your area and they can do the upgrade for you. My installation has been running trouble free for about 2 years and the speed has been fantastic. If you do decide to go this route, any SSD would work and then use the install kit OWC has available. The video instructions are:

https://eshop.macsales.com/installvideos/imac_mid27_2011_ssddiy/

Re-Read the specificiations. If they say Up to 550 MB/s that indicates it's not an absolute. At any rate you will see a dramatic increase in speed when the computer boots, shuts down and opens large files.

You need to post more info, Mid 2011, with how much memory and hard drive size?

The 21 inch models came with a 500 gig drive, and 4 gigs memory.

While you can add a second drive, it requires major disassembly.

Large capacity SSD's are expensive, too...

I would add memory first, an additional 16 gigs, to 20, will help greatly. 4 slots, two are available..

Data Memory Systems, http://www.datamemorysystems.com/apple-imac-intel-core-i5-quad-core-2-5ghz-21-5- mc309ll/a-mid-2011-memory-upgrades/

Second, upgrade the small hard drive. I put a 2 terabyte Western Digital Black ($99 Shell Shocker sale) in my mid 2011. Hard, but not nearly as hard as a second drive. You will need the temp sensor, OEM drive had it built in, replacement does not.

https://eshop.macsales.com/item/OWC/DIDIMACHDD11/

An internal SSD as either primary or second would be my final move. Most cost, most disassembly..

TIp : Do not replace the DVD drive with a SSD. It will work but is will not be as fast as replacing the hard disk or adding a cable to the motherboard and putting the SSD behind the DVD drive. Get the cable kit on eBay.