The IE-proprietary AlphaImageLoader filter attempts to fix a problem with semi-transparent true color PNG files in IE versions less than Version 7. However, this filter blocks rendering and freezes the browser while the image is being downloaded.
Additionally, it increases memory consumption. The problem is further multiplied because it is applied per element, not per image.
YSlow recommends:
the best approach is to avoid AlphaImageLoader completely and use gracefully degrading PNG8 instead, which are fine in IE. If you absolutely need AlphaImageLoader, use the underscore hack _filter as to not penalize your IE7+ users.
Feel free to contact us with your suggestions, links or ideas!