Third party driver vendors can crap out a graphical login pretty trivially.

If your driver messes up DWM.exe it can blow your whole compositing service and draw every window in software mode which is, without exaggerating, 10000x slower than Direct Draw/GDI back in the day.

Ever played an old Direct Draw game on Windows 8 and up? It will crawl. The game can only be played with WineD3D, which is basically a Wine build for Windows and it has ddraw.dll hoooked to its own DirectX or OpenGL libraries. This is the only way to get these games running fast again.

On software rendering for the whole desktop, the same. A text mode browser (or Dillo, or Links2) can be runable at useable speeds before downloading an updated driver or patch.

By the moment you try Chromium or Firefox with no acceleration (even under current machines), when you have to run both DWM.exe, the rendering process and desktop done with software rendering in CPU, it will slow down like hell because on top of that it will run some JS crap in the browser for granted on vendor driver pages.

Maybe a current i7 can withstand it, but not for long as browsers love to spawn threads and eat RAM like crazy. Add a 4K display and say goodbye to performance.