App Windows opening slow / dissapearing

Macbook Pro (2019)

Mac OS Big Sur 11.2.3

I have this extremely frustrating and weird issue where windows from every app sometimes take minutes to load; going from Finder windows; to the About this Mac window; to Photoshop.... everything.

It also happens a lot for apps that are in the background; the window just disappear; when i tab or click the app to the foreground; there is no window (but when i right click on the icon, there is a list with active windows, i just can't see/open them).

I did a SMC reset; ran Onyx; DXDrive; virus scan; i can't find anything wrong. There are no error reports or logs in console.

Last thing i want to try is a clean install; but no body likes doing that... i was hoping there would be some other cause or solution.