I've been working on adding a voxel framework to our engine for use in sims where we need destructible terrain / rock. The first project that will use this technology is a continuous miner simulation. Although the framework has been in place for a while, it has only become stable (bug free and efficient) in the last week or two. I've had to do a fair bit of profiling in order to eliminate bottlenecks and boost performance (I use a tool called Luke Stackwalker, which I quite like).
I had a gloomy moment when I first integrated it into the sim because there was a noticeable frame rate drop, but it turned out to simply be some debugging / debug visualization code I had left in. After commenting that out the framerate went right back up to a point that was actually better than anticipated considering (a) how much collision detection logic I'm running, (b) that the state of the system is being propagated through the system via state packets and (c) that I was running both dynamics and graphics on the same PC (so twice the load). It cuts through the wall pretty smoothly and the framerate is currently above 60fps.