Though I am sort of late on the off-topic argument, I'll input my own experience as I currently have a low-end (duo-core).
I can play Half-Life 2 on 30 fps with pretty high settings (excluding AA).
I can play Blockland on ~14 fps with relatively high (for the game) settings + minimum shaders.
That's it.
On topic, it appears Despair Syndrome achieves to force darkness to people without shaders, with some sort of "sphere of vision", and beyond it pitch black "fog".