can I add memory to a late 2015 iMac?

A non-Retina 21.5" model, where the RAM was not just sealed in but soldered in.

The good news for the OP is that even the cheap versions of that iMac had 8 GB of RAM.

The bad news is that the OP probably has a 1 TB 5400-rpm notebook hard drive, or a 1 TB Fusion Drive with only 24 GB of flash storage. One of those might be dragging down speed more than the amount of RAM.

The low-end CPU (1.6 GHz dual-core, vs. at least 3.3 GHz quad-core in other 21.5" models released at the same time) wouldn't help, either.

No. That CPU speed was only used in a 21.5-inch model.

(247071)

gopher57 wrote:

Thanks for sharing your expertise. As for the bad news you mentioned, could either of those weak links be improved in a cost-effective way?

You could improve on a slow startup drive by getting an external USB 3 / SATA SSD, and making it your startup drive.

As for the CPU, I think you're stuck.