Can I add memory to my iMac Retina 4 K, 21.5" 2017 that has 2 memory slots?

That Mac can officially take up to 32 GB of RAM (2 x 16 GB), and can actually take up to 64 GB (2 x 32 GB).

Macs can be picky about RAM, and now that Crucial is no longer in the business of selling Mac RAM upgrades, I would suggest getting RAM from Other World Computing.

Other World Computing – OWC Memory Upgrades For 21.5" iMac with Retina 4K & Non-Retina Display (2017)

However, that 21.5" iMac is not designed to be user-serviceable. Opening it and resealing it is a job perhaps best left to a repair shop.

You may also want to use Activity Monitor to check whether you need more RAM. Often, with 21.5" iMacs, the cause of poor performance is not lack of RAM, but the use of slow mechanical hard drives or of 1 TB Fusion Drives that include a measly 24 – 32 GB of SSD space.

Mrg36 wrote:

Thank you, do you think it would be worth it to try to upgrade this Mac for what I use it for? I guess I should ask whether the APPLE HDD HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM) can even be upgraded?

You're welcome.

The HDD and RAM can be upgraded, but I would not consider it for two reasons.

1) That iMac is sealed and must be completely dissembled to upgrade.

2) It's not worth it to spend that much money on an 8 year old iMac.

At this point, it looks like it is working fine for what you do the way it is.

If you want to improve that iMac, your best bet is to run from an external SSD as Allan suggests.

The other option is to replace that old iMac with a new 24" iMac or optionally a new Mac mini and an external display.

What you want to do is to run Activity Monitor (Applications > Utilities > Activity Monitor) with the Memory tab open while you are doing things that might use a lot of RAM.

Then you look at the area at the very bottom – the Memory Pressure graph, and the numbers next to it.

The Memory Pressure graph is color-coded. Green means the Mac thinks you had enough RAM for what you were doing. Yellow means the Mac thinks RAM was marginal. Red means the Mac thinks you had insufficient RAM, and that this was hurting performance.

Swap Used here was 0 bytes - which means that the Mac was never so pressed for RAM that it had to use the SSD or HDD to slowly simulate more.

If you have lots of Greens in your Memory Pressure graph, and your Swap Used is also very small or zero, a lack of RAM is not your problem.

The current RAM usage in that screenshot looks OK.

The next step is to find out if the iMac has a software or hardware problem.

To do that, download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post back here with your Report, as per > How to use the Add Text Feature When Post… - Apple Community

That 2017 21.5" iMac is an entry level model with a really a slow 5400rpm HDD.

Restarting the iMac every few days, instead waiting for weeks will help.

Beside those two things, your EtreCheckPro report looks OK.

To follow up on den.thed's wise information, your slow-spec factory hard drive is performing about as fast as can be expected:

Performance:

System Load: 1.25 (1 min ago) 1.33 (5 min ago) 1.42 (15 min ago)

Nominal I/O usage: 0.05 MB/s

File system: 43.37 seconds

Write speed: 71 MB/s

Read speed: 72 MB/s

70-80MB/sec is as fast as they moved data when new. Therefore, no amount of RAM can help that. Ever.

File system is a rough gauge of drive health. Yours is still good, in the healthy range but appropriate to its age.

Den.thed's advice to restart more often is spot-on. A restart resets memory and zeros the Swap value. With 8GB RAM, a restart every 4-7 days is prudent.

If you are not ready to buy a new computer, the most cost-effective DIY way to see a performance improvement is to set up an external solid-state boot volume:

Use an external SSD as your startup disk … - Apple Community

The least expensive incarnation of that option will improve your Write/Read speeds to 400MB/sec. This solution does not require the sealed computer to opened.