Too slow iMac late 2015

Take it in to get the drive replaced. Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. Also, see How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD).

Besides Kappy's sage advice, something is using too much RAM...

Virtual Memory Information:

Physical RAM: 8 GB

Free RAM: 18 MB

So maybe opt for an SSD instead of the HDD.

BDAqua said: "So maybe opt for an SSD instead of the HDD."

DEFINITELY opt for an SSD instead of the HDD.

That alone will noticeably improve the performance of the machine.

Open Activity Monitor, sort on Memory, any clues?

OK thanks, must be to do with the failing drive.