ram for my iMac

RAM won’t help performance much. If you are having performance issues, then I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

Crucial and Other World Computing (aka macsales.com) are the top two rated venders. Both have lookup tools on their website to direct you to the correct RAM, but you can double check the specifications from Apple here: https://support.apple.com/en-us/HT201191

A faster HD, i.e. an SSD, will speed up boot time, application and file open and save time. But not processing time once the file is open unless you're very low on ram and are using swap files for RAM.

Additional RAM will allow you to have more apps and files open without memory issues.

Definitely run Etrecheck and post the report. There could very well be some 3rd party software that's contributing to your problem and the report can help us determine that.