The space theme continues - nice one! As for more extensions, it'd be nice if the blocks speed up a bit as you go (it starts off at a nice speed but could do with getting a bit faster.) Also, if you constantly move left and right you can hold the block in the same position and cheat while you work out where to put it!

There appears to be a delay after a piece lands and stops moving. This delay should be due to the creation of a new tetris piece, but there is no delay on the scenario inside my computer, so i will be constantly updating things onto the gallery and testing them there.

With regards to the performance issues, have you declared your images as static fields or are you re-loading / re-initialising them each time? That may well be the issue - refactoring your code so the images are all static (and therefore just loaded once in the lifetime of the game rather than each time a new block is made) may help with the speed up factor :)