Slow iMac 2017--do I need more RAM?

Most likely the very slow 5400 RPM hard drive. But please download and run Etrecheck  It is a diagnostic tool that's very useful to us in finding problems. Also it will give us further specs on your Mac. After it runs post the log file here. It will contain no personal information. Allowing full drive access will improve the quality of the report.

To post the log file click on share report and use the Page icon in your reply window.

The hard drive replacement on the 21.5" iMac is a PITA, so I'd suggest having that done professionally. Also replacing the slow rotational hard drive with an SSD will make a dramatic difference in speed.

Be sure to make a good backup, (Time Machine or clone) if you do not already have one.

As we've now had a second confirmation of a failing drive, those login items just fell far down the list of concerns.

However, It is easy to turn them off. System Preferences > Users & Groups > {your account name} > Login Items tab. Use the "minus" button

to remove those from the list. This does not remove the apps from your computer, ; it only stops them launching when you start it.

Like the others, if your drive must be replaced, I recommend an SSD. Apple won't upgrade--they only replace "in kind." You'll need to find an independent shop that is comfortable working on sealed-case iMacs. It is not a task for "do it yourselfing" unless you are comfortable inside computers and are good at manipulating tiny and easily-borked connectors!

Want some real-world numbers for what an SSD can do for you? I suffered the same slow 5400, 3GPps drive as your in an older MacBook Pro and its best read/write speeds were about 65-80MBps. Here are its EtreCheck values now with an inexpensive 6GBps SSD:

Performance:

    System Load: 1.61 (1 min ago) 1.60 (5 min ago) 2.94 (15 min ago)

    Nominal I/O speed: 0.15 MB/s

    File system: 28.20 seconds

    Write speed: 487 MB/s

    Read speed: 482 MB/s

It was like getting a new computer.

As nearly as I can tell, Full Drive access is not available with OS 10.13 so don't worry about it.

You have one "cleaning" app running and it is older than your computer:

[Running] com.syniumsoftware.CleanAppDaemon.plist (? ef658ad4 - installed 2015-03-25)

These are completely unneeded and interfere with the excellent self-maintenance features you paid Apple to install in the Mac OS. I have seen EtreCheck reports where removing a cleaning app also removed warnings of a failing hard drive.

You also have high RAM-usage apps auto-launching at boot in your User Login Items. With only 8GB of hard-to-upgrade RAM, you really cannot afford that. At least remove Word, Acrobat, and Safari. If Magic Menu and ZoomOpener are system mods, they need to go too.

However the hard drive is, as in most 21.5-inch iMacs, the basic bottleneck. Your read/write scores are unusually low and likely indicate HD troubles:

Performance:

System Load: 1.84 (1 min ago) 1.83 (5 min ago) 1.84 (15 min ago)

Nominal I/O speed: 0.80 MB/s

File system: 120.00 seconds (timed out)

Write speed: 31 MB/s

Read speed: 35 MB/s

We have a 2011 iMac with the same slow 3GBps 5400 rpm mechanical drive and its scores significantly exceed yours:

Performance:

    System Load: 1.62 (1 min ago) 1.67 (5 min ago) 1.87 (15 min ago)

    Nominal I/O speed: 0.05 MB/s

    File system: 38.28 seconds

    Write speed:  112 MB/s

    Read speed:  91 MB/s

Remove CleanApp per the developers' instructions, restart your Mac and run EtreCheck again. if those read/write scores go up to nearly 100, your computer should run better. If not, the hard drive is truly the problem, and we can address that next.

Clean app is in /Library/LaunchDaemons folder.

Zoom opener and Magic Menu are in Login Items. Go to System Preferences > Users & Groups and use the minus (-) button to remove them. But I doubt that will help a failing hard drive.

Etrecheck does not always report a failing hard drive correctly. Check with Drive DX which is a much more through check of your hard drive.

https://binaryfruit.com/drivedx

See what Drive DX reports.

You're welcome, good luck!