RAM HELP

Unfortunately, the 2015 21.5" iMacs have soldered on Ram that cannot be upgraded at all. (There is no way to upgrade RAM through software. Ram is a physical hardware chip). See here: Install memory in an iMac - Apple Support

You may be able to improve performance by cleaning up any startup items or running processes that may be slowing it down. Normal culprits of this are Antivirus Apps, Malware apps, Mac cleaning apps.

You can download Etrecheck and perform a scan, and post the report here for us to look over and see if there's anything we cna suggest to improve performance on your iMac.

See here: Using EtreCheck - Apple Community

I agree, RAM is what is needed, but it unfortunately can not be added. In addition to what was noted, there are some things you can do which may help to improve things a bit. They are “maintenance” steps which are often ignored by many people.

·      It is important to occasionally restart a Mac. Ideally, once a week or more often. This does some system cleaning.

·      Regularly clearing the history of your browsers can help as all the data is stored in RAM. Clearing frees up RAM for system use. The Chrome browser uses a lot more RAM than other browsers do.

·      Some people seldom empty their trash. If you haven’t, then do so. Also empty your email trash.

·      Some programs use a lot of RAM. (Like many Adobe apps.) When using then, close any other apps not being used.

·      Doing a restart in the safe mode can clear a number of problems. This is done by holding down the shift key at start up. This process can take up to ten minutes while the computer is doing its maintenance and cleans the cache. You may then restart normally.

·      It is also a good idea to periodically run the Disk Utility app. If it reports some issues, then rerun it repeatedly as needed until the final report is ok.

·      Do not function with a lot of apps open if you are not using them.

·      Last, and most important, as noted, you should remove any and all Mac cleaner and anti virus programs. They will cause problems and will significantly slow your computer.

I hope these suggestions are helpful and enable you to speed up your computer somewhat.