Mac running pathetically slow.

tolska wrote:

I am thinking my only solution is to back up the system with Time Machine on a portable hard drive and then

restore the Mac to factory settings.

Does this mean that you currently do not have any backups?

Try running the third party app EtreCheck and post the complete report here so we can examine it for clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

etresoft wrote:

But if you have an older 21" iMac, it might still have a mechanical hard drive installed. That hard drive may be failing or just horribly slow. There's no easy way to fix that, other than buying a new Mac.

Agreed, except macOS can be installed onto & run from an external USB3 SSD (best if it supports UASP as well) since the iMac would have USB3 ports which would provide better performance than an internal Hard Drive. As long as the internal hard drive failure is not severe enough to cause performance issues even when booted from external media.

tolska wrote:

I am thinking my only solution is to back up the system with Time Machine on a portable hard drive and then

restore the Mac to factory settings.

Then reinstall all the applications I use.

Would those with more knowledge than me advise me to do this?

Making a backup is definitely a good idea.

However, I don't know if reinstalling from the backup will help. You can certainly try if you want.

But if you have an older 21" iMac, it might still have a mechanical hard drive installed. That hard drive may be failing or just horribly slow. There's no easy way to fix that, other than buying a new Mac.

tolska wrote:

I am an author so use it for writing in MS Word, emails etc.

All the more reason to make a backup.