I think 3-5 seconds for everything is a bit much.
However, I see where you're coming from. In the time frame that you mention I too have noticed a drastic slow down. I would imagine it to be a combination of:
A) The increasing amount of users online. This taxes EVERYTHING from the local regional carriers to the main network backbones to the server hardware to the already sketchy DNS system.
B) Websites not seeing the need to scale as a priority over adding new features. Websites that grow very quickly seem to sometimes follow this trend.