Nice game idea, but this gets very laggy for me whenever I shoot something / get shot. What are you using for the explosion? If you're creating new images rather than using the same static ones this might be causing the problem, or alternatively it could be the sounds playing.

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 :)

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!