Speed Issues

I am new to max, and was working on this, with the end result hopefully being something where explosions underground will affect the "top soil".'

Strict
Global objects:TList = New TList
Global walkability[800,600]
Const move_speed# =1

' used by fps code
Local old_ms=MilliSecs()
Local renders
Local fps

Type dirt
	Field x#,y#,xPos,yPos
	
	Function start_game(pheta#)
		Local Width% = 800, Gravity# = 9.8
		Local V0# = Sqr(((width/2)*Gravity)/(Sin(pheta)*Cos(pheta)))
		Local T# = 2*V0*Sin(pheta)*.5
		For Local i# = 0 To T Step .01
			Local x# = V0*Cos(pheta)*i
			Local y# = 600-((V0*Sin(pheta)*i) - (gravity*i*i*.5))
				While y < 599 And x < 799
					dirt.create(x,y)
					y = y+1
				Wend
		Next
	EndFunction 
	
	Function create(x#,y#)
		Local temp:dirt = New dirt
		temp.x = x
		temp.y = y
		temp.xPos = Floor(temp.x)
		temp.yPos = Floor(temp.y)
		walkability[temp.xPos,temp.yPos] = True
		objects.addlast(temp)
	EndFunction
	
	Function _Delete(x,y)
		For Local temp:dirt = EachIn objects
			If temp.xPos = x And temp.yPos = y Then
				walkability[temp.xPos,temp.yPos] = False  
				objects.remove(temp)
				Exit 
			EndIf
		Next
	EndFunction 
	
	Method update()
		If walkability[xPos,yPos+1] = False Then 
			
			walkability[xPos,yPos] = False
			y:+move_speed
			If y > 598 Then y = 598
			xPos = Floor(x) ; yPos = Floor(y)
			walkability[xPos,yPos] = True
			
		EndIf
		Plot x,y
	EndMethod
EndType

Graphics 800,600
dirt.start_game(60)
 Repeat
Cls
For Local temp:dirt = EachIn objects
	temp.update()
Next

If MouseDown(1) Then 
	For Local i = -20 To 20
		For Local k = -20 To 20
			dirt._delete(MouseX()+i,MouseY()+k)
		Next
	Next
EndIf
renders=renders+1

' calculate fps
If MilliSecs()-old_ms>=1000
	old_ms=MilliSecs()
	fps=renders
	renders=0
EndIf
DrawText fps,10,10
Flip
Until KeyDown(1) Or AppTerminate()
End

...It is SLOW ...Any Ideas?

Also, is there a way to sort items in lists? It would be nice if the soil would all move together when the layer beneath it is wiped out...