Major slowdown creating more than 600 particles.

I found your problem. You were calling the recycle method for each star, but there's also a loop for each star in the method. So for 400 stars you were doing 120000 interations! I do also think your aliens are also suffering from a similar situation.

Just put a call to the recycle method inside the loop in the update function and remove it from your main loop.

Here's the changes I made:

[code]
Strict
'image made global within type: slight speed increase
'star list global within type: very lisght speed increase

Const GR_WIDTH = 1024, GR_HEIGHT = 768, DEPTH = 16, HERTZ = 60

Graphics GR_WIDTH,GR_HEIGHT,DEPTH,HERTZ

SetMaskColor 255,0,255


Type star

	Field xpos:Int
	Field ypos:Int
	Field speed:Int
	Global starImg:TImage
	Global starlist:TList
	Global num_stars
	Global max_stars

	Function create_star()
		num_stars = 0
		max_stars = 500
		starlist=CreateList()
		For Local x:Int = 0 To max_stars-1
			Local s:star = New star
			ListAddLast starlist,(s)
			s.xpos = rnd(0,GR_WIDTH)
			s.ypos = Rand(-1,-GR_HEIGHT)
			s.speed = Rand(2,15)
			num_stars:+1
		Next
		
		'create star image
		SetColor 255,0,255
		DrawRect(0,0,3,3)
		SetColor 255,255,255
		Plot 1,0
		Plot 0,1	
		Plot 1,1
		Plot 2,1
		Plot 1,2
		starImg=CreateImage(3,3)
		GrabImage(starImg,0,0)	
		
	End Function
	
	Function update_stars()
	
		For Local s:star = EachIn starlist
		
			s.ypos:+s.speed
			s.recycle_star()
			s.draw_star()
			
		Next
		
	End Function
	
	Method draw_star()
		DrawImage starImg,xpos,ypos,0
	End Method
	
	Method recycle_star()
		If ypos > GR_HEIGHT Then ypos = -1
	End Method
	
End Type

Const TARGET:Int=60
Local frames:Int=0
star.create_star()
Local frameTime:Int
Local totalTime:Int=0
Local tCounter:Int=0
Repeat
	frameTime=MilliSecs()
	Cls
		SetColor 255,255,255
		star.update_stars()
		DrawRect 0,0,160,28
		SetColor 0,0,0
		DrawText TARGET+" Frames:"+totalTime+"ms",0,12
		DrawText "Stars:"+star.num_stars,0,0
		tCounter:+(MilliSecs()-frameTime)
		If frames=TARGET Then
			totalTime=tCounter
			frames=0
			tCounter=0
		Else
			frames:+1
		EndIf
	FlushMem
	Flip
	
Until KeyHit(KEY_ESCAPE)