Why does procedural background slow down UIScrollView?

Why does procedural background slow down UIScrollView?

My problem code is in a procedural background that I am drawing in a
UIView (drawRect:) which I am then adding as a subview to UIScrollView.
The procedural code is below. It draws box shapes, which look sort of like
a skyline. Any Ideas why this is slowing down the my UIScrollView? It
seems to only be slow on the first scroll, then its as if it is cached.
The background can be as much as a thousand pixels wide or more at times.
See image...
- (void)drawRect:(CGRect)rect
{
UIBezierPath *vertLine = [[UIBezierPath alloc] init];
[vertLine moveToPoint:CGPointMake(0,self.frame.size.height)];
int detail = 10;
int ranNum = 0;
int count = self.bounds.size.width/detail;
CGFloat heightIncrement = 0.0;
CGFloat minHeight = self.frame.size.height;
CGFloat xPos = 0;
CGFloat yPos = self.frame.size.height-20;
for (int i =0; i<count; i++)
{
ranNum += (arc4random() % 9)-5;
yPos -= (arc4random() % 30);
[vertLine addLineToPoint:CGPointMake(xPos,yPos)];
xPos += (arc4random() % 20)+10;
[vertLine addLineToPoint:CGPointMake(xPos,yPos)];
yPos += (arc4random() % 30);
[vertLine addLineToPoint:CGPointMake(xPos,yPos)];
xPos += (arc4random() % 30);
[vertLine addLineToPoint:CGPointMake(xPos,yPos)];
if (yPos>self.frame.size.height-10) {
yPos = self.frame.size.height-10;
}
if (yPos<self.frame.size.height-50) {
yPos = self.frame.size.height-50;
}
}
[vertLine addLineToPoint:CGPointMake(count*20,(self.frame.size.height))];
[[UIColor colorWithRed:0.0/255.0 green:38.0/255.0 blue:51.0/255 alpha:1]
setFill];
[vertLine fill];
}