OP:
If you're not willing to spend any money, you're NOT "getting more speed".
Understand that before going further.
If you have USB3 ports, you could buy a low-cost external SSD, plug it in, and set that up to be the "new boot drive" for the time being.
I'd suggest a $10 enclosure like this:
SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)
tinyurl.com
Then, get a 2.5" SATA SSD (I'd suggest 512gb or even 256gb) for it.
I'd recommend Crucial or Sandisk.
Something like this:
Start your system in seconds store up to 4TB of data and upgrade with an SSD you can count on Join more and more people who are keeping their family videos travel photos music and important documents on an SSD and get the near-instant performance and lasting reliability that comes with solid stat...
tinyurl.com
Or, you could get a 256gb for even less.
Erase it with disk utility (either HFS+ or APFS, depending on what OS you're using).
Then, either install a fresh copy of the OS, or use CarbonCopyCloner to "selectively clone" the OS folders, app folders, and account (you may have to leave some things behind).
Get it set up and the iMac will run MUCH better (particularly if it has only a platter-based hard drive inside).
Do this, and you'll come back here and tell us,
"I never would have believed that something so easy to do would speed it up so much!"...