You might want to check to see how much RAM the computer has. That model can be upgraded to 6 GB of RAM and based on my experience using the same model iMac, it will run much better.
The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I had 6GB installed on an early 2008 iMac supposedly limited to 4 GB and noticed an improvement.
Crucial
Other World Computing
Use the combo update to save time in Software Update.
10.6.8 Combo Updater
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
El Capitan 10.11 Compatibility information
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.