Can someone tell me why this is sooooo slow?

Function DoIris()
	Local x = 200,y = 150,d = 1000
	Local center_x = 400,center_y = 300

	Local circle:TImage = CreateImage(800,600,1,MASKEDIMAGE | DYNAMICIMAGE)
	GrabImage(circle,0,0)

	Repeat
		d = 1000
		Repeat
			Cls
			
			SetColor(255,0,255)		' The mask color
			
			x = center_x - (d / 2)	' Convert from silly 'circle in box' to more conventional circle center
			y = center_y - (d / 2)
			
			DrawOval(x,y,d,d)		' Draw the circle on the back buffer
			d:- 5
			
			GrabImage(circle,0,0)	' And then copy it to circle:Timage
			
'			DrawLevel()				' Now, draw the game level (This is NOT the bottle neck here...)

			SetColor(255,255,255)	' Since current color affects drawing operations...
			DrawImage(circle,0,0)	' Now draw the circle image we created on top of the level so it will show through
			
'			Delay(30)

			Flip					' Show what we've got
		Until d <= 10				' Loop again when we've got a really small circle
	Forever
End Function

Let it run for half a minute or so and it will bring your entire system to a screeching halt (even after you stop it), requiring a restart.

Also, since the mask color is 255,0,255 (same as the color of the circle), shouldn't it show the items behind it through?

What am I missing here?

Thanks,
Russell

p.s. Even with DrawLevel() and Delay() commented out, it still crawls...