Latitude 5490 Flickering and slowdown

I opened the laptop and started doing... Nothing. I just installed GeForce now. And the UI éléments started lagging to show up (Windows 10 LTSC). For example I clicked the start menu and it showed like 1 second later. The UI didn't even flinch. But then it did show up. Anyway.

In Linux(I'm dualbooting), it's flickering like shit. Even worse.

Closing/reopening the laptop fixes the issue for 1 minute then it's doing this stuff again

Note that the laptop is running only off AC POWER and no battery is installed (it was cooked so I removed it).

Thanks.