I believe I have a GPU problem on iMac

On 2012 and newer iMac, an GPU repleace is a logic board replacement. The vidoe hardware is integral with and soldered to the logic board. Not " slots." for a gfx card.

Hi,

I have similar iMac, RAM is little bit bigger, though it's running normal.

You'd better try to run Apple Diagnostics if it reports Graphic related error.

Use Apple Diagnostics to test your Mac - Apple Support

That iMac has fairly decent spec's.

However there are a number of things that can cause it to be slow.

To check get and run EtreCheckPro,

from > https://etrecheck.com/en/index.html

Then post back here with your EtreCheckPro report,

as per > How to use the Add Text Feature When Post… - Apple Community

Memory is not balanced…

BANK 0/DIMM0 - 16 GB DDR4 SO-DIMM 2400

BANK 0/DIMM1 - 4 GB DDR4 SO-DIMM 2400

BANK 1/DIMM0 - 4 GB DDR4 SO-DIMM 2400

BANK 1/DIMM1 - Empty

Bank 0 or Bank 1 should contain both 4 gb chips….

You have them split between Banks…

You're welcome.

macOS has Built-in Security, see > macOS - Security - Apple

Your system is bogged down by unnecessary third party junk-ware.

CleanMyMac and Avast need to be completely uninstalled as per the developer instructions.

You also have Proton and Open VPN's, which are useless unless you can live with slow buggy internet.

After you have uninstalled those from your iMac, startup in Safe Mode followed by a normal restart,

see > How to use safe mode on your Mac - Apple Support

Then run and post another EtreCheckPro report.

The reported speeds are low for a Fusion drive in a 2017 iMac 27:

Performance:

System Load: 59.25 (1 min ago) 86.05 (5 min ago) 99.00 (15 min ago)

Nominal I/O usage: 0.13 MB/s

File system: 121.26 seconds (timed out)

Write speed: 470 MB/s

Read speed: 985 MB/s

Still, that is fast enough that the computer should never "feel" or report as slow.

I collect EtreCheck drive reports and found one for the same model iMac with the same Fusion drive specs. Its score is hugely faster:

   Write speed: 766 MB/s

   Read speed: 2687 MB/s

That leads me, like others, to suspect CleanMyMac. I have seen Etrecheck reports where evicting CMM improved drive scores significantly.

Most client-side VPNs like you have do more harm that good.

I would also test with the WD Passport for disconnected. WD drives are great but the little enclosures they use were designed to go on sale every other weekend, not provide top performance. It would not hurt to test with all other external drives disconnected.

What makes you think it's GPU problem?

I see clean my mac and avast loaded…