Help with slow response time and GPU issue

Hi there, so I bought this 2012 iMac and upgraded the original version by installing an Samsung 870 EVO 2.5" SSD and Crucial 16Gb of DDR3 RAM in it. I upgraded macOS to Sonoma 14.1 through OPCL on the SSD.

Everything was running perfectly fine until around 2–3 weeks ago, when the mac became very slow all of a sudden.

I shut it down (the load time to the lock screen was much longer than usual). I tried resetting PRAM, cleared my SSD, and did a clean reinstall of MacOS Sonoma.

It was once again fine for a day or so before it went back to its previous issues.

The wallpapers don't load, there is extreme stuttering when loading and changing through apps, and when playing videos, I sometimes see the image being distorted (reminding me of static TV lines). I think these issues may have to do with the GPU, but I am not 100 percent sure.

Before the issues, I was using it to do some Matlab and coding, but I'm unsure if I can do with the amount of lag it has now.

What's weird is that I can write normally on webpages, but on the actual browser search bar, it's super laggy.

I will add the images of the specs, as well as a some images of the issue.

Thank you for your help.