So I wrote a custom view which displays a list of bitmaps. The way this works is to draw a grid of images by loading them from disk, then drawing those images using Canvas.drawBitmap(Bitmap, Rect, Rect, Paint).

And during scrolling it was dog slow.

So what I did was to pre-scale the images in a weak hash map:

    private WeakHashMap fScaledBitmaps = new WeakHashMap();

    private Bitmap getScaledBitmap(String url, int cellWidth, int cellHeight)
    {
        /*
         * Check our cache and return if it's present
         */
        Bitmap bmap = fScaledBitmaps.get(url);
        if (bmap != null) {
            if ((bmap.getWidth() == cellWidth) && (bmap.getHeight() == cellHeight)) return bmap;
            // size different; kill bitmap
            bmap.recycle();
            fScaledBitmaps.remove(url);
        }
        
        bmap = ... get our image from interior cache ...
        if (bmap == null) {
            // bitmap not present; return null
            return null;
        } else {
            // Bitmap loaded. Now grab the scaled version
            Bitmap scale = Bitmap.createScaledBitmap(bmap, cellWidth, cellHeight, true);
            bmap.recycle();

            fScaledBitmaps.put(url, scale);
            return scale;
        }
    }

And this sped up scrolling from sluggish drawing once a second to quick and smooth.

Lesson: drawing a scaled image to a canvas is frighteningly expensive. Like an order of magnitude slower than pre-scaling the bitmap and storing it in a weak reference or weak hash map.