What is the maximum memory supported by the imac 18.1 model (imac 21.5 2017)

An Apple Store will not upgrade the Memory and install an SSD.

Also, to verify this is the issue, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

➡️ Make sure you enable Apple security. Please go to System Preferences > Software Update, and turn on the following:

• Check for Updates
• Download new updates when available
• Install App Updates from the App Store
• Install System data files and security update

Then, go to System Preferences > Security & Privacy > General, and make sure the "Allow Apps downloaded from" is set to either "App Store" or "App Store and Identified Developers" (that may be the only options shown).

➡️ Re-enable Apple System Integrity Protection!

➡️ You also have no Time Machine backup. For Time Machine, you need an external hard drive. Apple sells external hard drives, such as the LaCie Mobile Drive External Hard Drive USB-C USB 3.0 - Apple, LaCie Rugged USB-C + USB 3.0 Portable Hard Drive - Apple, or the G-Technology G-DRIVE mobile USB-C Portable Hard Drive - Apple. Regardless of which drive you choose, it needs to be formatted. Follow these steps:

1. Go to Disk Utility (Finder > Go > Utilities)
2. In Disk Utility, select “View” > “Show All Devices”.
3. Select the top and non-indented drive in the external section of Disk Utility that matches the name of your external drive. 
4. Click Erase
5. Name: any name you want
6. Format: Use Mac OS Extended (Journaled) if you are using macOS Catalina or earlier, or APFS if you are using macOS Big Sur.
7. Scheme: GUID Partition Map
8. Click Erase

Finally, setup Time Machine: Back up your Mac with Time Machine - Apple Support.

➡️ Delete AppCleaner.

➡️ Other than that, the reason your iMac is very slow is due to the 5400 rpm hard drive. Very slow. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Cheers,

Jack

What you recommend, an external drive or a internal one and some more RAM?

The external drive is the most cost-effective and beneficial thing you can do for now. It may let you limp along with this model until in the position to replace it with a regular iMac with a pro-grade processor.

RAM will not significantly help what you are seeing today and is expensive to install.

1) First, this is the crippled education/institutional iMac with an older and slow 2-core laptop drive.

iMac Model: iMac18,1

2,3 GHz Intel Core i5 (i5-7360U) CPU: 2-core

The next model up had a minimum 3.0 Ghz, four-core processor better suited to a desktop workstation. IMHO this iMac is under-powered to use Creative Cloud. We can't fix that.

2) It has the slow, under-spec hard drive as Jack and I suspected:

disk0 - APPLE HDD HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM)

Internal SATA 3 Gigabit Serial ATA

The best transfer speeds you can expect form that drive unde rthe best condition are about 80MB/sec. Yurs is a bit behind that:

Performance:

System Load: 1.52 (1 min ago) 1.65 (5 min ago) 1.77 (15 min ago)

Nominal I/O speed: 12.82 MB/s

File system: 57.08 seconds

Write speed: 66 MB/s

Read speed: 68 MB/s

likely due to having a couple of self-inflected wounds: uTorrent installed and a "cleaner" App. Torrent software runs actively in the background and you don't have enough "background" to spare.

Cleaning apps interfere with elegant self-maintenance routines Apple builds into the macOS, and can slow hard drive performance; those third party apps interfere with what you've paid Apple to include.

Cat-like, Macs clean themselves.

3) Youneed to restart a computer with 8GB RAM more often than every ten days:

System Software:

macOS Mojave 10.14.6 (18G103)

Time since boot: About 10 days

I recommend every 2-3 days. That simple act will make your 8GB RAM go farther without the questionable expense of throwing in more RAM just because RAM sellers claim it speeds up things. That will reduce who much data Virtual Memory has to write to a slow hard drive.

4) There is an external SSD workaround that will help with speed without taking apart anything. If interested, please post back for details.

To answer your first question, 32GB RAM (2x16GB) is the maximum. However I would strongly advise against attempting that upgrade since a complete disassembly, including logic board removal, is required just to access the RAM slots. It is very much not user serviceable.

Instead I'd suggest simply getting an external USB-C SSD and reinstalling MacOS onto that.

(Although for advanced or frequent adobe creative cloud work you may do better just buying a new higher tier iMac).

You cannot upgrade the RAM on that model of iMac. Only an AASP might.

Why are you wanting to upgrade the RAM?

Jack