Just a thought, but perhaps you could create a separate PImage object to quickly capture a given frame from the PGraphics to the PImage (using PGraphics.copy() I believe). This copy() operation would be called from within the draw loop, after which point you could use your separate thread to call the PImage.save() method to write the PImage data to disk. This way, the frame capturing is still in sync with the draw loop, but it shouldn't slow down your animation because you're still using the other thread to perform the relatively slow task of saving your images to disk.

Of course, if the saving operation is so slow that it begins to fall seriously behind the rate of frame captures, you'll run into some nasty concurrency problems. In that case, you would want to keep an array of PImage objects available to be written to, cycling through them as necessary. This process would be similar to the internal memory that digital cameras have, using a fast-write RAM buffer when the relatively slow flash memory becomes a bottleneck.

Hope this makes at least a little sense!

greg