Flash - Possible To Improve Performance Of Drawing Technique?

Am currently working on a tool created by a colleague of mine and would like to improve performance. Basically, it's a tool for drawing on screen and uses a combination of Sprites and the Graphics class to draw a line segment every time the mouse is moved when the mouse button is down. Here is the drawing bit:

// on MouseMove
protected function drawLine(e:MouseEvent):void {
if (currentTool.thickness > 0){
//pen
var line:Sprite = new Sprite();
line.graphics.lineStyle(currentTool.thickness, currentColour);
[Code] .....

As you can see, it checks if the line 'thickness' property and draws if it is and erases if it isn't. I did think of using a technique similar to blotting here where it'd draw to a bitmap but I'm not sure this'd give the performance boost I want or if, indeed, there would be any way to have an eraser function. The drawing itself works nicely - this isn't the problem, it's the performance of the subsequent 'drawn' sprites.