iMac runs slow and beachballs constantly

It is also the entry level dual -core processor:

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

However, most of the answer is right here:

Performance:

System Load: 1.56 (1 min ago) 1.63 (5 min ago) 1.75 (15 min ago)

Nominal I/O speed: 3.93 MB/s

File system: 48.01 seconds

Write speed: 46 MB/s

Read speed: 31 MB/s

Those speeds are below normal for that model. Should be in the 50-70MB/s range, and those would still be painfully slow.

You have the option with that model to get a USB3 external drive enclosure and install in it a 6GBps SATA solid state drive (SSD). You clone your HD to the external SSD and then use System Preferences > Startup Disk to set the external as the boot volume. That will run the read/write speeds up to about 400MB/s and make a real difference in how long things take, like booting and app launch times.

Sadly, nothing much shows there other than the standard slowness causers of 8GB RAM & 5400rpm drive, so I'd run Drive DX to check the HDD health.

OK, let us know the outcome please.

It's sad, I definitely am afraid to even think about a newer Mac anymore.