It’s good to have your iMac dust cleaned every 2 years by a Mac service technician, so if your iMac is opened anyways, why not to replace the slow hard drive with a 10x faster SSD?
Sounds logical. Well, there’s another option without the need of disassembling your iMac, that you can do on your own:
Buy an SSD that comes with USB 3, or the more expensive version > buy an SSD that comes with Thunderbolt.
For example the 250GB Samsung T5 costs 113 EUR it has an USB 3 port link
The stylish 240GB Transcend JetDrive has Thunderbolt but it costs 256 EUR link
What’s the difference besides the price?
macOS (OS X) does not support the TRIM function as of yet over USB. Why is TRIM support important?
If you have upgraded your computer with an SSD and you run it for months, most likely you will experience a slowdown if TRIM support is not enabled and your SSD is filled with data to it’s 80% capacity. We have dealt with lot’s of MacBook Pros and iMacs like this that were taken to an independent Apple repair shop for SSD upgrade, but the technicians did not enable the TRIM function. Because of this the read and write speed have slowed down to 35-65MB/sec from the expected 450-550 MB/sec, resulting a frequently spinning beach ball to be seen. So an SSD in a USB casing is not the best option on longer run.
The Transcend JetDrive family is an ideal solution as it comes with Thunderbolt that supports passing the TRIM command to the SSD.