IMac is super slow, start up especially. Report attached

The three data points that supports the others' spot-on comments about the hard drive is right here:

Performance:

System Load: 1.77 (1 min ago) 1.84 (5 min ago) 1.44 (15 min ago)

Nominal I/O usage: 6.73 MB/s

File system: 73.60 seconds ⚠️

Write speed: 58 MB/s ⚠️

Read speed: 56 MB/s ⚠️

Even when the drive was 100% healthy, its Write/Read values could not exceed about 70-80 MB/sec with newer macOS versions, and that is still dreadfully slow. Adding RAM, even were it easy (it's not), will have zero effect on drive performance.

Your drive may be struggling with general heath because of the 70+ second file system score. I have a collection of EtreCheck drive scores, and only drives that are close to failure post File System scores over about 40-50seconds.

However, the "Pachyderm on the Premises" is right here:

iMac Model: iMac18,1

2.3 GHz Dual-Core Intel Core i5 (i5-7360U) CPU: 2-core

You have the crippled educational/institutional iMac, intended for price-sensitive bulk sales where computing power is a second- or third-order factor. It has a slow 2.3 Ghz laptop-class, 2-core processor.

The standard consumer entry-level iMac model that year, the iMac (Retina 4K 21.5-inch, 2017), came with a proper desktop-class 3.0gHz 4-core process as the SLOWEST option. The makes a huge difference on bench scores, as shown in this comparison from the MacTracker database (yours is on top):

That is quite a performance penalty for a US$200 cost savings.

Still, the external SSD option already offered will make a considerable difference in how you experience your computer. The least expensive incarnation will increase drive scores to about 400MB/sec, and requires neither professional installation nor opening a computer case that Apple did not design for consumer opening.

as already mentioned, you have a very slow HDD. you could have an SSD installed internally. but an easier and more cost effective option would be to get an external SSD and install macOS onto it. then you could boot from that drive.

to implement that, please see Use an external SSD as your startup disk with your iMac or Mac mini - User Tip.

There is a mechanical drive present in your Mac. Mechanical harddrives tend to get slow after a period of time.

You might get some more performance by getting a SSD upgrade done.