Chris Campbell, from the Java2D team, has posted a very nice blog entry about ImageIO performance improvement in Mustang (aka Java SE 6). He shows off the amazing perf increase from 1.4 to 1.5 to 1.6 b62 to 1.6 b63. If you are still sticking with 1.4 it's high time you switched!
I recommend you to read, if you haven't done it yet, the Desktop Java in Action presentation where you'll find interesting numbers about startup speed and overall performance between 1.4, 1.5 and 1.6. Do not hesitate to use our weekly betas to ease your transition to 1.6. Thousands of people are already running Mustang every day and by the time it ships, it should be the best version of the Java SE platform ever. Also, if some bugs are still annoying you, do not forget you can contribute bug fixes! Your fixes have a high priority, higher than *our* fixes. An excellent way to make your life easier and give back to the community.