What? A shader on every object, blocks and entities, and multiple post processing shaders work fine for me (above 300fps).

Eventually depends on the complexity of the shaders you are using. The forward lighting shaders you find on the wiki are the hell if you use them extensively.
There is no nativ deferred lighting support in GS.

as far as i know, the current example shaders are nice to have, but their performance is..not so good.