in the new 1.7 snapshots the speed at which chunks load drastically decreases when Vsync is enabled, although this should usually rather increase the speed since the game doesn't need to calculate so many frames and has more ressources for loading the chunks.
it Also depends a lot on the framerate, although it should rather be seperated so that chunks don't take ages to load in laggy areas, but just lag a little more in the beginning untill all chunks are loaded.

The attached video shows the bug in more detail.