[Scratch the first part of my original post. The memory on this iMac is soldered in! The only correct part of my opening paragraph was that "the maximum possible amount is 16 GB – and you're already there."]
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Are you sure that lack of memory is the problem? What does the Memory Pressure graph in Activity Monitor say, if you watch it while running everyday workloads? That graph is color-coded, so that
- Green indicates your system has plenty of RAM
- Yellow indicates that the amount of RAM is marginal for what you're doing
- Red indicates that you're overloading memory (and hurting system performance)
I'd be more inclined to suspect problems with what you have installed on your Mac, or issues with the speed of the drive. Many of those Macs came with slow 2.5" 5400 RPM SATA hard drives. Some came with 1 TB Fusion Drives that had 128 GB of SSD space; others, with 256 GB or 512 GB of flash storage (i.e., a SSD).
If you have just a 1 TB hard drive, then an external USB 3.0 / SATA SSD startup drive might be just the thing to make the Mac seem a lot faster.