Looks like an implementation of "where am I pointing at" is using the renderer and is causing a pretty major graphic stall.

JVisual VM:
https://dl.dropboxusercontent.com/u/9117298/bc_renderer.png

Source of the trouble:
https://github.com/BuildCraft/BuildCraft/blob/NextGen/common/buildcraft/BuildCraftCore.java#L440

Thanks for the finding. This code should only be called on demand then. Will fix.