If the iMac has a platter-based hard drive, that may be what is "making it slow".
The drive just can't handle the demands of the OS.
But... there's a relatively easy and cheap fix:
You can buy a USB3 SSD, plug it in, and set it up to become the boot drive.
This is easy to do -- ANYONE can do it.
I would suggest getting a Samsung t5 or a Sandisk Extreme USB3 SSD -- small and compact.
Get a 1tb version. Costs more than a 500gb, but things will be much easier.
Then...
Connect the SSD to the iMac.
Use Disk Utility to erase it.
If you are using High Sierra or Mojave, erase to APFS with GUID partition format.
If you are using Low Sierra or earlier, erase to Mac OS extended with journaling enabled, GUID partition format.
Then...
Download CarbonCopyCloner:
www.bombich.com
CCC is free to download and use for 30 days.
Use CCC to "clone" the contents of your internal drive to the SSD.
It will take a little while.
Then, go to the startup disk preference pane and set the external SSD to be the boot drive.
Then, reboot.
It should boot and run much faster!