updating the screen would.  calculating takes very little time, its just mainly conditionals "if M=1:O+1-->O:End"

Not true, when you're checking collisions of multiple enemies with multiple bullets, it really strains the calc for speed, that's why you noticed slowdowns in the first version when there were more ships.  Even the sprite drawing calculations become negligible compared to the collision detection.

Actually, you can detect collisions every other frame (as long as things are moving slow enough) for the best speed increase without changing anything else.  You can use that Z=0 trick for that too.

Don't feel like you have to have to follow all the advice everybody has offered so far.  Some of it is redundant, like you wouldn't want to move by 2 pixels AND update every other frame, becasue that would be 4 pixels per frame, way too fast. And doing calculations twice before updating is the same thing as updating every other frame.  Just go for simplicity.  There's many many ways to increase speed.

ok, I tried to make it a little bit faster, I'm kinda happy with the way it looks now, but its a little bit jerky in the movement.  What it does now (in psuedo-code) is:
Initialize
Repeat Health=0 or getkey(15)
For(Z,0,3)
Draw enemies
Draw all bullets
Pause 25-[number of enemies]*8/3
If Z
move
End
End
Display special game over message

anyways, here ya go. 

Controls:
Arrows: move
2nd: shoot
Alpha: full speed
Clear: quit

Damn you, _player.
DAMN YOU.

*presses the CLEAR button*
*reads message*
I c wat u did thar.

  you're welcome

Screenshit

I've wanted to test this thing anyway, but now I HAVE to. I must know if pressing the clear button Rick Rolls me. Speed looks a LOT better in the current screenshit DJ <s>Omnomnom</s> Omnimaga posted. P.S. Oddly enough, I *may* have some old lobster sprites for a Space shooter game I was working on years ago.