slow hard drive, can anyone help?

You have a 5400 RPM mechanical drive which is the lowest one to use. With each OS update the drive is being tasked more and more. You only have 8GB Ram but not much file swapping so probably not that. You can try to clone your Mac to an external SSD and use as your prime boot drive that should improve your speed.

The (under-)performance of the iMac 21.5” HDD model gets discussed fairly often around here.

Above reply is correct, an external USB 3.x or Thunderbolt 3 SSD is about your best option, short of replacement.

See the following thread for more details of the usual suggestions > iMac is too slow, Apps are opening up ver… - Apple Community

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 ~60 times faster for storage, depending on what external SSD you buy. For more info, instructions, and what external SSD to buy, please read: https://discussions.apple.com/docs/DOC-250003583.

Cheers,

Jack

I don't see the usual suspects in the report. The main problem is that the model you have is the slowest Mac offered by Apple at the time of purchase, 8 GB of RAM, i5 CPU and a 5400 rpm hard drive.

There's nothing you can do about the RAM but there is regarding the drive. You can get an external SSD and clone your boot drive to it with Carbon Copy Cloner and boot and run off of it.

Your current Read and Write speeds are

Write speed: 45 MB/s

Read speed: 41 MB/s

With an external SSD you can get Read and Writes speeds of about

which is about 10 times faster that what you currently get. This will affect booting, app and file opening and saving speed, i.e anything that has to do with disc access speed.

These OWC SSDs are excellent choices for external SSD:

Envoy Pro EX with USB-C or USB 3 - best from a speed vs price standpoint

OWC Envoy Pro EX with Thunderbolt 3 - fastest if you have a Thunderbolt 3 port but more expensive

I suggest you contact OWC customer support to find out which would be best for your particular model and situation.