IMO: I would rather have built in (multipage) TIFF support at the ImageIO level in stead of in JavaFX. TIFF has its uses, especially the multipage thingie makes it unique compared to the more popular image types that the JDK supports out of the box.
I never experienced the JAI logic to be slow though. At least the platform independent version I was taking from the Maven repositories does the job decently in my opinion.
I once had an issue with image handling beeing very slow. It happened for all images which contained a color profile. After storing the images again without color profile image handling was fast again. Just an idea :-)