My iMac is running so slow

The first problem I see is you have a base model iMac from 2015 (now 7 years old!). These machines were inexpensive and were designed to do just e-mail and minor web surfing. They were not designed to do any type of "heavy lifting". The primary bottle neck on these machines is the 5400 RPM hard disk which is glacially slow when it is operating in it's prime. To check the drive's health download the free version of DriveDX (DriveDx - the most advanced drive health diagnostics and monitoring utility (binaryfruit.com)) and run it through a test. If the drive shows any errors that is very bad.

However, after testing if the drive is okay then an acceptable option is to invest in an external SSD and then clone the internal HD to the new SSD and use the SSD as your boot drive. This will transform a slow performer to a decent performer. To save money I'd recommend the OWC Mercury On-The-Go Pro 1TB SSD Storage Solution (macsales.com) and a 5V 2.0Amp Barrel Style AC Power Adapter for OWC Mercury On-The-Go (macsales.com).

That being said, you are investing about $235 in a 7 year old computer. Another option is to sell the current Mac and upgrade to a new 24" iMac with the M1 processor. If you decide to go that route get one with 16GB of RAM and a 1TB SSD and you will have a rocket of a computer compared to what you have now.

We need to see your EtreCheck report, telling us you ran the report is of no value if we cannot see what may be the problem. That info would be revealed by the report. Please click https://discussions.apple.com/docs/DOC-250000211and follow the steps.

Your welcome.