Feedback on gameloop with sprites

Hi!
Okey, so I have managed to make three sprite sheets which I am very happy about (everything is new to me, so every success is a big success for me).

The sprites are enemies that will spawn outside the right side of the screen and transition and disappear on the left. My code works, but I don’t know if it’s the right thing to do regarding performance etc.

if event.time - timeLastPlane \>= math.random(4000, 5000) then  
  
  
 -- Randomly position it on the top of the screen  
 stringHeight = math.random(20, 190)  
 require "sprite"  
  
 --Random nr  
 local r = math.random(1, 3);  
 local sheet  
  
 if r == 1 then  
 sheet = sprite.newSpriteSheet( "plane1.png", 226, 116 )  
 planexscale = 0.5  
 planeyscale = 0.5  
 thetime = 5000  
  
 elseif r == 2 then  
 sheet = sprite.newSpriteSheet( "plane2.png", 201, 95 )  
 planexscale = 0.5  
 planeyscale = 0.5  
 thetime = 5000  
  
 elseif r == 3 then  
 sheet = sprite.newSpriteSheet( "plane3.png", 225, 125 )  
 planexscale = 0.3  
 planeyscale = 0.3  
 thetime = 4000  
 end  
  
 local spriteSet = sprite.newSpriteSet(sheet, 1, 4)  
 sprite.add( spriteSet, "plane", 1, 4, 200, 0 )   
  
 local instance = sprite.newSprite( spriteSet )  
 instance.x = 520  
 instance.y = stringHeight  
 instance.xScale = planexscale  
 instance.yScale = planeyscale  
  
 instance:prepare("plane")  
 instance:play()  
 instance.name = "plane"  
  
 physics.addBody(instance, "kinematic")  
  
 transition.to( instance, { time=thetime, alpha=1, x=-420, y=stringHeight, onComplete = function(self) self.parent:remove(self); self = nil; end } )  
  
 enemiesLayer:insert(instance)  
 timeLastPlane = event.time-1500  
 end  

Any feedback would be appreciated!

Best regards,
joelwe [import]uid: 54640 topic_id: 9813 reply_id: 309813[/import]