If you have an iMac 9,1, 20" of 2009 that is very slow, you really need to install a SSD inside. The computer was as fast as new after replacing the old HDD (25 000hours of use).
The unmounting of the iMac is quite easy, you can follow some tutorial on Youtube. But the difficult part is the installation of the SSD.
Indeed, you will ned a 2.5" to 3.5" SATA adapter that is very small and sized for the imac or the screen of the iMac won't properly fit.
Be also carefull with the SATA connector from the motherboard because they are really short.
I had to find a DIY solution, bending some parts and use plastic ties.