I realize that one of the shortcomings of the late-2015 iMac is that it tops out at 8 GB RAM, and that this probably contributes to the slowness.
Don't be quite so sure. it is RAM Yes, it's true that the RAM in a 2015 iMac 21.5 cannot be upgraded even by the deities on Mount Apple, but the real cause in slowness in most entry-level iMacs is not RAM but a slow, under-spec mechanical hard drive that was the base-level storage in that model. If the computer has a factory Fusion drive or factory solid-date drive (SSD), it should run fine to at least Catalina.
My some is running an older and slower Mac than yours under Catalina with 8GB RAM and it is fast. However, we upgraded the slow hard drive to an aftermarket SSD.
Signs that it is the hard drive and not RAM are;
- the computer is slow to boot
- apps are slow to launch
- once launched, apps run fine
People can and do throw hundreds of dollars of RAM at what is a hard drive problem and still be surprised to see no improvement. If it is the hard drive, there is a cost-effective workaround that would csot about US$100 to implement, and can be moved to a newer Mac once you decide this one is done.
I won't thought out that recommendation without data, however, Unfortunately, I can only surmise from model history and thousand of posts here. To have us develop a specific diagnosis of a computer we can neither see nor touch, we need to see performance data. Fortunately there is a safe, secure way to do that.
We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:
https://etrecheck.com/index
The free version will do nicely for this purpose, although the app is worthy of our financial support.
We can see hard data about drive performance, software issues, 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:
