Most of you probably know this trick. Keep the 'working/terragen' folder empty and the tiles in another folder. When you have an updated tile you want to add simply MOVE it into the terragen folder (do not copy and paste). Import terragen textures as per normal - it will do it very quickly. When done move the tile back to the other folder.
This saves vast amounts of time. Only the new tiles need to be read, not every single tile, hugely speeding up the process. By moving the tile rather than copying and pasting if you also save processing time. Neat!
This only took me two years to figure out! Nobody can accuse me of being quick off the mark!