iMac (Retina 4K, 21.5-inch, Late 2015) with only 8GB.

In the 2015 iMacs there is absolute no way on Heaven or Earth to increase RAM without replacing the entire logic board. In that year's iMacs, RAM is soldered to and integral with the logic board. No RAM slots. 2012-2014 and 2017-current iMacs with 21.5-inch display require a service provider to upgrade RAM. Not made user-accessible.

If you want an iMac whose RAM is easily accessible, you have to get the 27-inch model.

👉🏻 However, based on hundreds of reports here, your slowness is likely not RAM but the dreadfully slow factory hard drive in those models. It is rated slower that the hard drives Apple used in in 2011 entry-level iMacs.

Still, we need to eliminate software causes of slowness first.

To check for softare issues and to help with the hard drive diagnosis, please post an EtreCheck report. We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck, available free from the Mac App Store here:

https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12

We can also see hard data about drive performance and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find):

When its report displays, click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.

⚠️ Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

NOTE: Changes in late 2018 to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Additional Text” window: